Each track can be individually set to one of two play options:

1. Play over other tracks
2. Stop all other tracks from playing

This means you can nominate particular tracks which can play at the same time as any other tracks that might be playing. If no other tracks are playing, these tracks will still play normally. Tracks that are set to Play over other tracks will always obey if another track that is set to Stop all other tracksfrom playing begins playing.

Playing a track that is set to Play over other tracks.
If another track or other tracks are playing when you play this track, this track will play over the tracks that were already playing. Even if those other tracks were set to Stop all other tracks from playing.

Playing a track that is set to Stop all other tracks from playing.
If another track or other tracks are playing when you play this track, the other tracks will stop, even if the other tracks are set to Play over other tracks.