The board on the side of the MK where you plug the pad I/O ("JAMMA" harness) and lights (three other connectors) into. Audio and VGA are also ran through this board on early revisions of this board (~MK3/MK4). Extra test/service/credit clear buttons are also on this board.
Variations of this board have been in use since the MK3, and these variations are known to have subtle wiring differences. As such, swapping a JAMMA PCB between drastically different hardware generations (such as MK3/MK4 and MK6+) is not recommended.
As the PIU LX cabinet uses different connectors for IO, this board is no longer needed for this cabinet. MK10 kits designed to upgrade older cabinets will still include one, however, also including an adapter harness to go from the JAMMA PCB to the (also included) LXIO.