It’sa spacer, goes on the axle between the crank arm and bb. Some set ups seem to need it, others not.
Deore 12 speed cranks I bought recently came with two of them and the axle is too long without them but they’re larger in dia than the bb I’m using and look stupid so I’ve stuck a spacer behind the bb cup in the nds instead.
I’d assemble your bb and cranks and see if you need it and go from there.