Any PARTs from 9-16 cannot sound at the same time as any other PART. They're restricted to sounding only when selected which forces all other PARTs OFF. There's not a way to change this. With two (or more) simultaneous sounds played with local piano keys - you can only have access to 8 PARTs at a time (and only within the range of 1-8), not 12 PARTs at a time.

Ah, yes. In a different context, I knew that. 😉 So I guess I've found the one-in-a-million Phil error! So within a single Performance (assuming I don't want to do element-level editing to create compound sounds within a single Part), I am limited to 7 LH bass splits (bass sound plus choice of 7 RH Parts for the total of 8), those different combinations selectable either by switching the active parts, or setting up Scenes, the former having the advantage of RH sounds not cutting out when you switch from one to the other (and there are probably other pros and cons to the two approaches). Getting back to the thought that started this thread, 7 available RH Parts goes more quickly if you're using up multiple Parts to make a single sound available in multiple octaves. But these are still clearly useful techniques.