All that said, I have had the question "If you can record to the Layer Quick-Boxes anyway, why bother having a separate Song recorder?"
The answer is Looping.
A MIDI Song recorded in from whatever source can have up to 16 loops points assigned during record by special notes inserted into the song, or after the song is recorded via the LOOP button settings. Layers recorded will only loop on their whole length (unless re-triggered by the Layer Sequencer steps or external reset) whereas Song Loops can be any size (in bars) and be very long.
Layers are limited to 128 bars, whereas a Song could be 300 bars long. Songs assign the MIDI PC's (instruments if GM) which can be modified in the MIDI PUSH button, but Layers only parasite on whatever channel is used... not to say a PC can't be set while recording a Layer.

Songs also compress the "preamble" of CC's, PC's, and PW's sent by many MIDI players, DAW or otherwise, to be sent only upon load or pressing the MIDI PUSH button. This relieves the MIDI stream of trying to push 200 commands through the stream on Play Start which, if extreme, will cause a very latent start. Layers consider any MIDI command as the record start point so, if extreme, could cause latency anomalies on the loop point trying to re-send all of this data each time, even though I have done this, best keep it below 6 channels!

Song Loops also intelligently reset certain controls on a jump or loop, for example:
The loop happens in the middle of a guitar pitchbend. Without any intervention, the pitchbend would remain "stuck" wherever it was when the loop ended, and all following notes on that channel would be off. Layer recording would create another Layer or append to capture the remaining pitchbend, which would be a problem too. The fact that Layers are more for live recording, the user knows this, thus would use the pitchbend in a timely fashion.

The last reason for having "Song Record" is that one possible method of creating a new "masterwork" is modeling it around an existing song downloaded from your favorite MIDI porn site, then modifying it beyond recognition to skirt copyright laws (lol) to use with TRAX trackers, build on it with Layers etc.
This couldn't be done with Layers because, again, the required loop partitioning would be impossible.
Pressing the P & S buttons on the Sequarallel will arm it to record a Song only. This must be done before any Layers have been recorded (logical, but this may change in the future). For proper looping, it's required that the source actually sends a MIDI clock (24/96 PPQN) even though recording without MIDI clock by using a modular clock has yielded some very surprising results, especially with a classical composition.

This can all be done while in Song 00, and even sequences (like, say, a metronome click) added, then saved as song project >0. You can't save Song 00 using the "Save As" though. As a safety, it requires a special method of selecting "Save init", choosing "yes", and holding for 5 seconds. Any Song Projects created in Song 00 will have all of your settings applied every time.
If you decide to dump all of your initial Song 00 settings, there's a way to return to factory settings on it as well, without affecting any other Song projects.

So I came up with the idea of "FX Bundles" where each sequencer element would have X number of FX bundles to choose from. TRAX trackers, for example, each have 8 FX bundles called "NoteFX". A single TRAX sequencer (one of 40 per project) with it's 64 steps 4 lanes wide could hold as many as 256 notes. Say you want ratchets on every 3rd row, but only in lane 2, and the you want triples on the first 2 beats, and quads on the remaining beats. This would be very time consuming to set up with lots of copying ratchet FX and changing them for the latter part of the sequence, need I go on...

NoteFX bundles include all of the FX you might want to apply to a note in the TRAX tracker.  So in the above scenario, you would simply select a bundle to use, say NoteFX#1. Select a note, then use that to open the NoteFX menu, scroll to ratchet, make your settings of division and if it will have a  curve or micro-arpeggiator, and exit.
In the TRAX Editor you can highlight FX*EDIT to display NoteFX numbers applied to every note in the grid, select FILL=3 and SKIP=3, and turn on COPY to copy your note with the FX#1 applied (you'll see it on the grid) double-click to copy it, then double-click and scroll down to paste the FX# to every 3rd note in that lane and click.
To set rows after 2 beats with FX#2, which will soon have the othr ratchet settings copied into it), turn off PASTE and select the starting point to highlite the FX#1's on every 3rd note in the same way, but scroll the encoder up one to set them all to FX#2.
Open FX#2, copy from FX#1 and change the ratchet to 4. Done!

But what if I want to use the same TRAX sequence in the Play List but transpose every 2nd block up a fifth? (you may ask!)
There's quite a few ways to do this, depending on your desire to "multi-task" other elements:
PL*FX.
Double click on the block in the Play List and select "PL*FX" to open settings for that block. There are 7 PL*FX bundles per track to choose from, similar to NoteFX. Select one and set it's transpose parameter. Apply this block's bundle to other blocks as needed.
SongFX.
You could have one TRAX tracker block in the Play List and have Song Loop's FX bundles setting it's transpose and scale. This is an option in the track's PL*FX, but relies on the SongFX "Send to TRAX" option being on as well.
Automation.
If you record the transpose change to an automation in real time, or edit the automation list, then place that automation into the Play List, then this will achieve the same goal.
CV or CC Remote.
Another way is to control the main transpose value via CV input on CC jack 5 or 6 or the Clock Out jack if not being used.
Via MIDI, a quick and dirty way is via a MIDI Remote CC# assigned to the transpose of the TRAX tracker (or main)!

Layer QB Buttons also have their own FX parameters and can be controlled by Song Loop FX the same as above, but to a more intricate degree, and include Swing by default. TRAX can follow the swing as well if it is set to clock multiply (you can't swing on a master clock/down beat) but has it's own "Swing Map" which should have a video of its own!

This is why the Sequarallel has recording abilities (besides recording a Song) to both the TRAX tracker and MIDI Layers.
The TRAX tracker can be set up to configure piano keys to control a live recording method so it can be controlled remotely and be constantly added to, changed, or re-define note values without changing their positions in the sequence. This is fun and inspiring for sure! Because TRAX is still a tracker, notes added automatically quantize to steps (although this can be removed by allowing delays) .
The Layers can be added into over time, or immediately, and have controls that can be assigned also to special keys. For hands-on, this is desirable. There is a quantize setting for each Layer Button so it'll only sound awful the first time as it's recorded :)

Getting MIDI to CV can be easy or more involved. At the simplest, a CV MODE can be set so all 4 rows of jacks output from the same MIDI channel. Assuming this would be for chords to 4 VCO's, one would set the ORDER to STACK. Notes beyond the stack can be discarded, wrap or pile up on the highest row. If you want to output a different channel to your VCO's copy the first CV MODE into the next and change channels to that channel, etc.
CV MODE is simple, especially if using MIDI channels, but it can be complex too. Sources can be defined down to only a certain sequence, or shared with a MIDI input on a certain channel. Rows of jacks can output from MPE zones 0 and 1. Glide parameters can be applied to individual jacks, gates turned into a trigger, and even VCO image selected. The CC CV outputs can be velocity, any MIDI CC, internal events triggers, or envelopes and envelopes attenuated by velocities, bipolar, voltage range, speed etc..
These aren't required to know right away though!

I like to play it in when the inspiration hits, then tweak it after. The "playing in" part is so easy, the tweaking part so fun! What more could anyone want in a module? I only hope I can convey this in the upcoming videos and tutorials.

An example would be a Layer Sequencer block plays out in 24 bars then hits a Conditional block. This block is set so that the Song track must step to a new loop, and TRAX1 must step to a new block. The first Song Loop is 6 bars long, and 3 following loops are 8 bars long, make 30 bars total, while there's one TRAX2 block that is 12 bars long. At the entry to the Conditional on the Layers track, the Song track would be at the 2nd bar of the final loop, and TRAX2 would be at the start of the block. This will set one condition, but needs to wait for 6 more bars for the next Song loop block.

If the Conditional block's Bar Setting was 4, then it would play through, but if >6 bars could JumpTo another Layer track group.
If the TRAX2 track's block was 20 bars long, it would be at the 4th bar at the Conditional time so, even though the Song loop will step in another 6 bars, the TRAX2 condition won't be met for another 16 bars, thus Layer track would fall through to next block.

These conditional blocks can be interesting, but also useful in cases where loops are different sizes. Any unused conditions can be simply set to "Bars" which is a default assumed state of the block. The JumpTo can be any row in the Play List 0 to 63.
Throw in an automation block and things can get really interesting! 
Here's a good example: http://www.freshnelly.com/tunes2021/seq-condits.mp3
The longest group is the Song track at 16 blocks in the Play List, but with 3 Layer tracks and all 5 TRAX tracks with groups of 3 to 6 blocks. The Song track is controlling transpose & scales. 

This mode uses the set global TRAX record channel over sources such as MIDI or LINK input, a recorded Layer, or another TRAX tracker. Useful scenarios for this mode might be:
- Several pads assigned to play chords during a live performance, also recorded to a Layer to continue the sequence
- Another TRAX tracker stepping slowly to cycle through the chords, then progress to a different set in another TRAX in Play List
- Triggering a series of chords at the set step rate by hitting 1 key at any time.

An obvious consideration is what happens in this TRAX sequence is just one of several TRAX patterns in the Play List track?
Because this mode is global to that particular TRAX button, all sequences in that track will use the Note2Chords mode. This can be advantageous in that the particular part or group of the "Song" can have chords that work with it.

The disclaimer here is I haven't actually implemented this yet, but it seems like a really slick feature to add (yep still in the feature creep phase at this time!) If end up not adding it, this post will mysteriously disappear like a video on youtube!

Update: This can be done with Remote Notes (probably best as local list, "Song") programmed to set the bottom of the sequence. It doesn't work when not playing, but when playing works great! If the Size is set to 1, then a single Chord will play over and over.

So! I set the SongFX Swing up to 30% ( 50% is normal max, Sequarallel will go to 90% ) and lo and behold the recordings became perfect when quantized. SongFX, or Song Loops, can control the swing of everything sequencing. It was separate at one time but served no useful function that was so was changed.
Using Swing during a recording Layers is logical when hand-playing in MIDI, but can yield some surprising results when Layers are recorded from a sequencer such as the BeatStep Pro.

i.e. if quantizing to 1/8ths but the BSP is stepping in 1/16ths, every intermediate note (not on a 1/8) will either be behind 1/8 in the first half of swing, or advanced in the latter half. As the modular clock takes precedent, recording another sequencer not in the same timing can also create some pretty cool grooves. The Sequarallel doesn't use MIDI clocks step by step as one may expect, but it's own internal clock locked to beats. The exception is an initial Song recording, which uses the MIDI clock for an exact duplicate.

I try to make melodies & embellishments that will go along also, perhaps with a different cadence or maybe even dissonance, as long as it sounds bearable as a Layer on top of what is already playing. Once there are 5 or 6 Layer Letters recorded, I then go into the Layer sequencer and make new steps with some of the Layers removed, expanding or lessening the bar count per step so loops overlap.

It's surprising how this basically puts together all of the parts of a song, i.e. Transposing a step or two into a refrain with different MIDI channel or repeating notes with a SongFX, or even scaling via SongFX. This all happens very quickly and easily! I plan to make a video on this soon to demonstrate. External sequencers can also be layered as well. Ones with mute tracks/output are best or new Layers will continually be created. With this recording I did a combination of a BSP and hand playing. The percussion is both a TRAX tracker and some BSP percussion: http://www.freshnelly.com/tunes2021/seq-PL_Layers.mp3

 

These two sets of remotes can be turned on where if there is no Song remote assignment, the CC# or Note# might be found in the Global set. Either can be turned off. The Learn function is a polling style making it easier to learn from multi-axis controllers that may send less numerous messages from the unintended axis. This happens over 1 second. The parameter is simply selected to highlight, then you move the MIDI control or hit the note you want to assign to it. Notes are better for On/Off type controls, tap to turn on, hold for 3/4 of a second to turn off... but if the parameter is a value, the previous value will be assumed the held state, and a new value selected will be considered the value when the note is just tapped. So it's important to make an adjustment to the parameter if you're teaching it notes.

The point of this high point is to point out that the track remotes, ( which use a series of notes to set a primary Quick-Box Button action like solo, mute, record, gate FX etc, and another series of notes to apply to different tracks ) are not protected against the Learned Remote method. So in effect, one could have a note doing two different things. For the most part this is useless but setting a transpose when a certain track key is hit might be one possible use.

Even though solo & mute could be assigned a MIDI note, the track remotes are grouped together so perhaps more efficient to use. The TRAX Record Sequencer remotes are an entirely different set of controls  that can be on a different channel to have nothing to do with the Learned or track remotes. Confused yet? As many or as little of these remotes can be used, which is ultimately up to the user. Myself, I like a fair amount of functionality globally, but tend to use a different board of knobs for the more tweekie stuff like TRAX NoteFX.
The Sequarallel has two MIDI input ports that can be used for remote inputs, but must share the same channel. If the channel is being used by something other than remotes on one port, it can be blocked in the Main MIDI I/O section of MIDI PUSH QB.

This opens up a whole new world inside the Layer Sequencer as other Song Loop's FX like Mirror, Velocity Levels, Note Lengths, Note Limits and on un-masked steps in the SongFX sub-sequencer "kill Mirror", Level +, Octave +, Length variation, and re-triggering with unmasked steps.

I like to have two empty Song Loops in the Play List, each with different FX, then several Layer Steps of a different size than the Song Loops so they don't align, and various Step FX applied. Recording a 1 minute Layer build of a 4 bar chord progression on a keyboard can make an entire song that is both quick & easy, but also a lot of fun!

This was one motivating force to nudge this module toward completion, just to see what others will do with just this part of the Sequarallel!

The following link has a sample of this method in action. It's fairly subtle with the lead synth, but there's also a bend on the snare at one point. http://www.freshnelly.com/tunes2021/sequar-LayBendFX.mp3

Quantized Layer Button will also quantize the constant controller or pitch-bend movement. This should be kept in mind if combining with a Layer Button that also has notes. It does have the advantage of offering a glissando sounding bend though, as in this example:
http://www.freshnelly.com/tunes2021/sequar-quickLayerRec4.mp3

Also, as a true automation of remote controls in MIDI time, in Settings (Global) one can turn on LAYER > REMOTE:YES and any MIDI CC or Notes on  the remote control channel will be recorded and used to control internal parameters. This of course requires that the MIDI control or Note has been captured to a parameter in the first place, which it should anyway... er, to be a remote ;)
When the Layer Letter is played by a Step, then the automation will be sent as if you're tweaking the knob itself.

Additionally, a CV Input can be set to control any internal parameter from a TRAX NoteFX element like a step's tone in a micro-arp to soloing a track to selecting a scale to selecting a jumpTo group in the Play List.. With a bit of imagination this feature can be a powerful way to control pretty much any type of sequence you have running. The possibilities are endless!

Here is an example of using it to change drums in the TRAX tracker sequencer. 3 CV Remotes: 3 TRAX Drums

When recorded into a TRAX, keep Swing in mind if there is one as it may affect the step the notes end up in. If lots of swing and Layer recording, then it will sound perfectly normal until the swing is removed. Layer Quantization may also place the notes in the "wrong" place.

I tested this with an O_C module as a Sequenzer then fed the CV outputs back into the Sequarallel's CC5 & CC6 inputs set to play MIDI notes. This became a sort of sub-sequencer that could be triggered by pretty much anything. I set it to be triggered by the CV inputs channel in the Envelope menu, and by a specific Layer's notes in CV Mode which are few. Once the Layer started the sequence it would be retriggered until something else went Solo, at which point it would stop because it was no longer triggering itself. The sequence also follows the SongFX Swing, because of trigger timing.
I found this pretty interesting!

Because the [ORD:ARPEG] source is determined by the [CV MODE:n] jack source, any sequence can also use the Arp's. Each Arp can handle up to 8 notes at once so it can generally keep up with whatever goes in without dropping notes. Here's a sample of 1 TRAX (+ a percussion loop) and 1 Layer with simple chords played in on measures: sequar-Arp-LayTrax.mp3

As they do use a lot more bandwidth (3 times as much) using them with a modulated CV input is going to reduce room in the stream for other controls. That said, using them in TRAX NoteFX can be very interesting as NRPN's can modify some parameters in a synth that there are no CC's for. One thing that comes to mind is the arpeggiators in my Korg synth. With the clock set to input and a TRAX updating the notes of the arp, plus the occasional NoteFX happening because the mask aligns, some of the cadences can be quite astounding!

Further on that is NRPN's to switch patches in the Korg. This is done by simply making a TRAX tracker with a single note (that is basically muted) and assigning a NoteFX with the NRPN in it. This runs all the time. Then anywhere in the Playlist I want the patch to change, I attach a PLFX which in turn will modify that value to whatever it was when I put it into the PLFX list. This is just one thing that I have derived so, yep, pretty useful!

The 3rd lane has 3 notes sprinkled unevenly over the 6 steps with noteFX 2 assigned to them all. The only thing this noteFX does is set the note probability to 77% so some notes won't play, but still, of course, abiding to the 5 step mask sequence mentioned above.
The 4th lane has only 1 note which is assigned noteFX 3. This noteFX has 6 echoes space 1:00 steps apart and a micro-arp, offset randomly, with +7 and +12 transpositions in it. I threw in a glide for good measure.

After all of that, I set the TRAX 1A MIDI channel to MPE zone 0, then jacks in CV Mode to channel 2, 3, 4, & 5 respectively (these are the zone 0 channels). BTW CV Mode's source channel set to MPE doesn't work this way as that defines channels for a particular jack, so all will feed to 1 jack.
What plays is a series of changes in the sequence exploring every combination of the inversion without being repetitive sounding, even though it's only 6 steps long! After this I added a new block in the Play List to follow, some PLFX to switch the clock rate and double the minimum bars so each block has the same duration.

Drums! In TRAX 1 I had started with a 4 step sequence with only 3 notes and applied a similar noteFX scenario and pre-decided micro-Arp on 1 note. It varies nicely enough to sound like a much longer sequence! I am adding this project to the SysEx page as Demo 8 if you want to hear it or even download and check it out!

The latter's micro-Arp can be set up while playing the sequence even if the CV is assigned already and running as once the NoteFX menu is up, the CV can no longer modify the start point. This is especially useful if doing a second CV as you can hear how this/these notes will sound with that CV's modifications. It is a very interesting way to control the notes in a sequence because many can be changed at once!  I am adding this project to the SysEx page as Demo 9

While on the topic of Automation, recording an automation on the remote channel seems rather silly because  PLFX can do it better and in time. But, the automation could be a slow change to a CV Mode# or SongFX parameter that changes over long (long) periods, knob tweaks...
Also, if you're sharp, you could record an automation through CV2MIDI CC. (Hint, you must turn off remote channel while recording! ..return to channel after)

Setting the Remote Note to arm a block (as above) which has a PLFX attached that can modify up to 4 different parameters!
On the plus side two parameters that work hand-in-hand like, say, scale and transpose or TRAX bottom and size, can be simultaneously changed.
On the minus side, a track must be sacrificed if not part of the composition, i.e. if a TRAX 2 A Block is used to attach PLFX#'s to, then TRAX 2 must either have the desired sequence that will also be played, or be empty, jumping away from any TRAX 2 letter that is playing. Using the Automation track is an option, but only has 4 PLFX#'s available.
Another minus, or perhaps a plus, is that these remote modifications will only take place once the present block playing has finished. With TRAX though, the blocks can step very quickly so it's very suitable for this. On the plus side, this acts as a time quantization for changes so may be better depending on the application.

Simply go into the Layer Bar Step Sequencer # and select  a Step, scroll left & double-click to get into the Steps FX menu. You'll notice the new option "USE SONGCHANNELS:YES". Be affirming this, any presently stepped into SongFX (from the SongLoop track in the Play List) can have it's channels mask applied to the present Layers in the Layer step.

This is very useful for: a) Layers that were created with multiple channels re: from a sequencer, or b) Layers created from an MPE keyboard on multiple channels of zone0 or zone1.
Perhaps you want to remove the percussion from a multi-channel, or certain notes from an MPE (that's fun!). For single channel Layers, it's not much different than just not including the Layer Letter in a Step, but Song Loops could be stepping faster or slower than Layer's bar steps are so it still adds a dimension of variability even in this case.

The main reason I added this feature was because there were occasions when I wanted a fairly long passage to be thinned out, but didn't want the Layer Bar Step progression to be altered. It was the perfect way to do this! I tend to build a Layer QB based on variations of the same progression, i.e. imagine different blues instrumentalists getting the spotlight for 12 or 24 measures... They all must have a unique rendition of the progression to sound cool (ya!)

If you want picture-perfect sequencer recording, it's probably better to switch over and record it into a Layer instead, but TRAX can offer a running and changing sequence non-stop, and if CV input is scale quantized (SongFX Scale & Transpose options in MIDI PUSH) then transposed using Song Loop's SongFX, even an LFO(s) can sound good! An interesting experiment might be to use a random CV generator that pops out a CV change every so often and let it populate the grid over time. If the CV's are quantized to scales, or the CVin is scale quantized using SongFX, the sequence could even sound good!
All of this is done in the last part of Tips&Tricks#1 video at https://www.youtube.com/watch?v=9uuHKTQ5Xqw&t=1690s