Functions of Buttons on the Joysticks-docs

does anyone know what the buttons do on the top of the joystick? and is there a documentation we could see.

Specifically the top center button thing that you can move with your thumb what does that do?

Thanks in advance

and I am talking about the joystick that came in the kit

and if that button thing at the top can do more than 1 thing would you mind filling me in as to what it can do

Thanks in advance again

Here are some links to Cd discussions about the Hat and other joystick buttons.