Think you've pretty much got there. The up and down arrow are station search or preset up/down. Don't know myself about the R/T button. Can't be important- not used it myself.
My manual is in the car at Russ's at the moment. I have a PDF of the US manual - will take a look when I get chance to fire up my laptop.
Am guessing it's to switch control over between radio and the address book control for your phone if you have BT fitted. Much easier to use Siri/Cortana (or your favourites list) to find an address book entry and call nowadays so if that's what it is that explains why I haven't used it. Not that I'd use any - I don't have any friends when I'm in the car