Taken from TBBOAH:
J101 This jumper determines the high order address bit for Fat Agnus. In it's normal position, the high-order bit is A23; in it's other position, this bit is A19. The current Fat Agnus chip requires the A23 signal for proper management of the memory at $C00000. Future Fat Agnus chips may do things differently; this keeps the current board compatible with simple enhancements.
J500 This jumper is used to enable the 512k of RAM at $C00000 and the real-time clock. It is normally closed; opening it will disable both this extra RAM and the real-time clock.
Whoops, looks like Lokki posted similar info. This thread is like an adventure through the Bermuda Triangle! lol