The P-ROC was the original board developed by multimorphic as a replacement for the WPC and early modern stern MPUs. It uses switch matrixes and it interfaces directly with the dmd controller and driver boards and itself is controlled/directed by a computer.
The P3-ROC is indeed used in the P3. It is also a replacement for the P-ROC and far more flexible. It does remove the ability to interface directly with a DMD, so if you needed that ability, the P-ROC is what you need.
It interfaces with boards called SW-16 and PD-16 as well as PD-LED. The SW-16 allows for connection of up to 16 switches (not matrixed), split into two banks. The PD-16 allows connection of up to 16 drivers split across two discrete banks. You can have eight devices at 5V and 8 at 50V for example, or all 16 at 50V. The PD-LED interfaces with both parallel and serial LEDs, servos, and can support a large number of single color or smaller number of rgb LEDs.
Each of these boards can be chained together to support insane numbers of switches, coils, and LEDs.
The P3-ROC is also controlled by a separate computer.
There are many more production and homebrew games that use the P3-ROC vs. the P-ROC because of the expanded capability and extensibility. It's also smaller, so you can mount it where convenient, as well as the addon boards, so you reduce wiring.
The P3 and its modules use the P3-ROC (that's what it was developed for), and when I build a module, I just purchase the number of sw-16, pd-16 and PD-LED boards needed to control the functions on the module. It's really slick because of the in-built extensibility.
I've built traditional homebrew games, and started doing so right as the P3-ROC came out. I've also built P3 modules and am quite comfortable with wiring and so forth for that board set.