DVD Studio Pro 1.5: How to Assign a Video Stream to a Track

Tracks are sets of video, audio, and subtitle files along with markers, angles, and stories. Tracks can be linked to buttons, scripts, menus, slideshows, and other tracks.

Select the track, then choose an item from the Asset pop-up menu (in the Video area of the Property Inspector). You can also drag a video asset from the Assets container to the Tracks container in the Project View.