In the Motion Mixer, motions are placed on tracks, and the tracks are organized into trackgroups. In other words, each trackgroup is a holder for one or more tracks. Each trackgroup can be filtered so the tracks within it affect only certain parts of the biped, such as its arms or legs. Every biped in the Motion Mixer can have multiple trackgroups, each with its own selection of biped parts. See Adding Tracks to the Mixer and Filtering Mixer Tracks.