Making use of unused button on steering wheel

Not sure if this helps at but I recently installed a stereo in my 2015 prius, kenwood DNR476S. It had an options to keep and use the functions of the steering wheel with an extra hardware. It's called maestro rr, it allowed me to keep the button as is or it gave me the option to change the buttons function all together with out cutting or splicing. So that useless button could be anything with this.

On top of that I can add extra futures to the buttons like when I hold a button it does what I programed it to do, for example the volume lower button if I hold it. My music mutes.