the AT+ is an independent foot-switchable Boost, essentially creating a second channel and gain stage for the pedal. The boost side of the AT+ is a slightly dirty and overdriven pre-boost that allows you to hit the front end of the AT drive channel and achieve more saturation and definition for applications where you need more. Since the Boost is independent, it can be used alone to drive your amp or other drive pedals in conjunction with the AT+ drive channel.
