Here's an example under performance control for all 16 parts.

First, I have the MIDI jumper in place and I have 16 parts assigned, one to every channel.

Second, I have parts 1 - 8 transmitting to parts 9 - 16 respectively. Zone control is active.

In the first photo I have keyboard control activated for parts 1 & 8 only (they are lit), You can observe that parts 9 & 16 are also sounding (see the levels are up).

Note the "block" around part 9 and still all (4) parts are sounding. This is because the "block" is around an active keyboard control part (keys are lit). When the "block" is around an active part then all active parts will sound.

Photo #2 that "block" is moved to part #2. Notice that only part #2 and its corresponding part #10 are sounding. That's because it is NOT an active part (keys not lit) and no other active parts will sound.

Photo #3 I selected part # 7 (keys not lit but it's sounding-see the level indicator) but this time the corresponding part #15 is NOT sounding. That's because I have turned it off using the "assign #1" key. Photo #4 has it activated and now part #15 is sounding.

In photo #5 I activated keyboard control part #4 (keys lit) and now because the "block" is around an active keyboard control part then all keyboard control parts are sounding. Those are 1, 4, 8, 9, 12 & 16

In photo #6 I have activated parts 6 & 15 and now 8 parts are sounding.

In photo #7 I have turned parts 4 & 6 off (keys NOT lit) but turned on part #2. However, only parts 4 & 12 are sounding because the "block" is around a non-active keyboard control part.

Photo #8 shows everything active and sounding. You can also select a part 9 - 16 like for a solo but you'll notice that the performance control light begins flashing meaning that you have temporarily exited the performance control mode since parts 9 - 16 aren't under keyboard control by default.

I went thought this exercise to see if I could control all the parts in some reasonable way.

What would be nice is that once a keyboard control part is deactivated (not lit) that another part between 9 - 16 could become active under keyboard control. You'd still only have a max of 8 parts available but in real time you could pull from the parts 9 - 16 as needed. You'd basically be trading one part for another part without having to switch performances.