From what I have found, but I have never used the "Back" button:
"It is a redundant button that allows you to go 'back' in menus rather than hitting the left arrow in the upper left corner. Or for example, when you are on the 'Radio' screen and go to Settings' the Back button will take you back to the 'Radio' screen."
Which button? You have your climate control for Auto and to control fan speed or shut the climate control off, Red and blue arrows to control temp, AC button to turn AC on and the recirculate button under that. The scroll knob to scroll through the stations/tracks, hazard button, and your button to partial disable traction control (the car skidding), the back button allows you to go back to the previous page on the Uconnect screen if you are on anything other than the main page.
Which button? You have your climate control for Auto and to control fan speed or shut the climate control off, Red and blue arrows to control temp, AC button to turn AC on and the recirculate button under that. The scroll knob to scroll through the stations/tracks, hazard button, and your button to partial disable traction control (the car skidding)
It does exactly that and also it works in other menu to go back. For me it's very useful button since I had a 128Gb SSD full of music with many folders