In the song structure editor you select the rhythm which will be used to generate the backing track.

Rhythm and Rhythm Parameters

Each song part can have its own rhythm. In practice however it’s difficult to have more than 2 or 3 different rhythms in one song because you’ll reach the 16 Midi channel limit.

Each rhythm defines rhythm parameters which let you adjust the way music is generated. Rhythm parameters can be adjusted on each song part.