You can load a single audio file or a set of stems. The benefit of stems is that they can be re-balanced during playback.
By default, backing tracks are played directly to the main stereo output (as defined in the Audio tab in Settings). You can also route the backing track into the Mixer so it can be processed using effects. This is achieved by setting the input of an Audio Channel Strip to the desired backing track rather than a physical input on your audio interface. You can blend the direct and processed signals as required.
Backing track audio files are copied into the Sessions Pro project file so they are they can be moved between devices without having to worry about external file references.