What do you call something that goes into a slot?
"Slot" here would be (according to Wiktionary):
A narrow depression, perforation, or aperture; especially, one for the reception of a piece fitting or sliding in it.
Edit for context:
I'm using the concepts in a software program I'm trying to write. In order to help the end users understand the end result, I came up with the physical representation of wooden boxes with an open top, called slots, with corresponding objects to go in these slots on a one-to-one basis. There would be many objects available per slot, but each object would only fit in one slot at a time