Key fob question

[SteveCZ28]

So I've owned my tahoe for a while. It came with 2 3 button remotes. I never thought anything about it. Well recently I was cleaning the dirt out of the unlock button on one fob, and tore a button off the rubber pad. So I bought 2 new shells and pads. Upon disassembly I noticed the board itself has 4 buttons on it. Which surprised me, that both boards are 4 button, but only 3 button shells. So my question is, what would that 4th button do?

 

[DenaliBQ]

trunk button as gm uses same remotes for cars and trucks at the time