[quotePost id=117905]it is surely an unnecessary limitation[/quotePost]
It's there, so it was necessary.
[quotePost id=117905]Using a DAW, the workaround would be to record the part using one Part on its channel, then copy that part on to other tracks and select in the DAW the MIDI output channels corresponding to the other layered Parts. That's far from ideal.

Have I understood this correctly?[/quotePost]
Not sure, your workaround/question is phrased in a confusing way and it's not clear what you want and what you suggest.

Depending on what Parts you think of using (Keyboard controlled 1-8 or the other 9-16), you can use Hybrid mode as well.
https://www.yamahasynth.com/learn/modx/mastering-modx-midi-settings-explained

also this might help:
https://www.yamahasynth.com/learn/modx/modx-midi-record-on-daw

Have I understood this correctly?

Not yet ...

Having your keyboard set to single-channel mode doesn't mean everything plays at the same time. It means your keyboard will be setup similarly to how other keyboards handle MIDI. They do not play everything at the same time either.

Each Part 1-16 would have note range limits that would determine which keys will trigger each Part.

Each Part 1-16 would have velocity range limits that would limit, of those above notes, which ones will trigger each Part.

Each Part 1-16 has arpeggio settings that would determine what triggers arpeggios. Similar to the above two rules but specific for arpeggios. Generally drums are controlled by arpeggios. They don't have to be - you can have each drum kit piece controlled individually by piano key press (instructed by the DAW). This does tend to limit your options some since drum kits are in a fixed range of the keyboard. Still, you have other key zones you could setup the other Parts.

The exact way you would deal with addressing one sound from another sound on other "traditional" keyboards is how you would address this on MODX while in single or hybrid modes.

Hybrid mode allows for you to have Parts 1-8 all on the same MIDI channel and then Parts 9-16 would be on their own MIDI channel. That gives you some amount of flexibility since maybe you place drums on Part 9 and then this wouldn't "tie up" that keyboard note range since Drums would have their own MIDI channel apart from Parts 1-8 which can all be combined.

All of the DAWs I'm aware of are fully capable of routing properly to MODX in multi-channel MIDI mode. There is some extra setup (vs single or hybrid) but still works. What I'm saying here is there's a possibility that adapting to a new workflow would be more beneficial. I don't have all of your constraints - so I'm not sure of this. However, it's generally true.

I guess it's worth also stating the "dark side" of this too.

"Most keyboards" will act like single-channel MIDI mode. There are exceptions. Kurzweil's flagships and other keyboards with 8+ zones. That said - the previous generation of keyboard (Yamaha Motif XF) had even more flexibility.

In Motif XF's Master Mode you could assign each "part" (I'll call them tracks herein) to any arbitrary MIDI input channel. Therefore, you could have two tracks assigned to the same MIDI channel. You could create 3 groups of 3 if you wanted to. 1:MIDI Ch1, 2:MIDI Ch1, 3:MIDI Ch1, 4:MIDI Ch2, 5:Ch2, 6:Ch2, 7:Ch3, 8:Ch3, 9:Ch3 ... and so on.

MODX is not that flexible like Motif XF was. The Motif XF really was a beast that checked lots of boxes. Today's implementation, in my opinion, is a step backwards. However, it's still as capable as "most keyboards" -- meaning those that support a single MIDI input channel.

Now this only becomes a limitation when your hardware setup doesn't support what you want to do. Generally this means connecting MODX straight to other gear without a computer/iPad between. In this case, you rely on the capabilities of the hardware on the other end which may not be able to address a sufficient number of MIDI channels for multi-channel mode (there's a hardware fix for this too - MIDI Solutions has a box that would "fix" this).

The computer itself and associated software is (should be - there could be "lesser" software out there ...) fully capable of handling the demands of multi-channel mode. Your workflow would be very similar to how standard GM MIDI is created - even older workflows.

Thank you for these detailed replies. Using the MODX as a sound source controlled by another synth (a Yamaha Clavinova piano, in my case) is more awkward than with most other synths or modules, it seems. If I were playing sounds from the MODX from a DAW (perhaps on Parts/MIDI channels 2-6) while wanting to play a split or layer from the master keyboard sending on a single MIDI channel is really difficult simply because the MODX does not (unlike all other multitimbral synths as far as I know) permit more than one part per MIDI channel. Unless channels 1-8 are used at the same time, apparently. I think this is a strange and unnecessary limitation.