Older switches didn't have that terminal - newer [replacement] switches do. I'm drawing a blank on what I concluded from it, it's been a while. Something up there between my ears says 'ground' but I'd have to check again to be sure. If this terminal is not common with any of the others, then maybe that was it. Would need to hook it to a volt/ohm meter again to refresh the memory.

Bottom-line is your car does not use it and you can ignore it.