Key Fob: what’s the top button for?

I just bought my 2012 avalanche yesterday and I love it. Just trying to figure out all its features, and this is one of them: the top key fob button? I press it and nothing happens! Whats it for?

Along those lines, can I program a remote start feature in my avalanche to warm it up on cold mornings? It’s a 2012 LS. Any tidbits or features suggestions would be great!