From FIXwiki

FIX Field 366

Tag	Name	Type	Enumerated	First introduced	Deprecated
366	AllocPrice	Price		FIX.4.2

Executed price for an AllocAccount (79) entry used when using "executed price" vs. "average price" allocations (e.g. Japan).

Notes

Please post comments, clarifications, examples here. Click on the edit button on the right and away you go! 

Don't forget to refer to the FPL Discussion Groups to back up anything you write.
You will often find the authoritative statements on matters of FIX specification interpretation there.
Links from FIXwiki pages to relevant discussions on the FPL site are very valuable.

For example if you were writing a clarification on how to specify the "quantity" of an order in FIX, you might add the
following relevant link, http://www.fixprotocol.org/discuss/read/f5014573, to back up your points.