I always thought it was for the Console adapter.
https://www.ultimarc.com/console.htmlThe site says the A-Pac and I-pac boards are universal and the I-pac info states the following.
The I-PAC2 has 32 inputs. They are all programmable and are marked as 2 joysticks, 8 buttons each, plus coin1, coin2, start1, start2 and Mame control keys. Any inputs can be assigned as a shift key to access an alternate code set.
This board also has a connector which can be configured to plug our trackball and spinner into, or for our console interfaces.