By default, the Motion Mixer expects that a mix you create will affect the entire non-biped object. If you like, you can Filtering Mixer Tracks a trackgroup so clips in the mix affect only selected body parts. This functionality makes it easy for you to assign one set of XAF files to control the motion of an object's upper body, while another set controls the lower body.
For more information and procedures on the use of trackgroup filters, see Filtering Mixer Tracks.
When on, highlighting an object automatically select all its children.