What da heck is this A500 side expansion board ???

[Tempest]

This card has 5 86-pol. A2000 CPU-Slots. You can use A2000 processor cards with it.

DMA Expander

[Tempest]

Yes, as far as I understand  you can use any A2000 accelerator board with this card, but only one DMA card at the same time. You can't use zorro cards, so no gfx card.
With an adapter you could also plugin a500 expansions.

5 CPU slots is a bit overkill, as you can only use one DMA card :-P