Video Track.
A new track is added to the version above the last existing track. You can re-order your tracks, or even move a track to a different version by dragging the track from the handle at the left of the track.
Mono Audio Track or New
Stereo Audio Track.
A new audio track is added to the timeline below the last existing audio track.
Mono Version or New
Stereo Version.
A new version is added to the timeline above the last existing version.