The 68851 is an external memory management unit (MMU) which is designed to provided page memory support for 68010 or 68020. It cannot be used with later versions of the processors such as the 040 or 060 because they have an internal MMU. It also cannot be used with EC versions of the 030, and possibly EC versions of the 020. Very few cards for the Amiga make use of the 68851 primarily because it can only be used with a small range of processors. Most Amiga's an accelerator cards use a processor which either has it's own MMU or is an EC version which cannot support an MMU. One of the few cards which does use this is the Commodore A2620.
Thanks to Joe Torre