When added to a belt, the buckle creates a prismatic socket on the item which has no gem color preference and accepts all gems except for meta gems.
It can be added to a belt that already has sockets.
Addition of further buckles to a belt will consume the buckle itself, however no extra socket will be added. Doing so will not destroy the gem in the current prismatic socket either.
Gems added to the socket will not help you achieve the color match bonus of the item, however they do contribute towards activation of a Meta gem.
This does stack with engineer tinker belt enhancements, such as [Frag Belt].