Insinkerator Sink Top Switch BISCUIT BUTTON STCBIS, is this the entire switch or just a replacement button?

This is the button only, for the complete kit you would want item number STS00 which comes with a chrome or a white switch, or STSS0 which comes with a chrome, satin nickel and white.  If you wan the biscuit button you would need to purchase this as well.