What do you call an object that goes in a "slot"?

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

Solution 1:

A key, or a token, depending on the context.

With the word key, I was referring to part of a machine component that is specially shaped to slot into, and lock with, the keyway of a shaft.

(Though, of course, a key more commonly refers to an object that slides in through the keyhole of a lock, and acts to unlock it.)

A token would be something similar to a coin, which would pass straight though a slot.

Solution 2:

In engineering it would be a tang

Solution 3:

Insert is a noun that might fit you (as for example in: shoe insert)