These tracks are called the overlay tracks because they are used to overlay
objects onto the Va/Fx/Vb base tracks. An example of the use of an overlay track
would be to put scrolling text in the video sequence. Each overlay track has
settings that define its transparency, enabling you to set a certain color, say
black, as transparent.
The default number of overlay tracks is 3, but this can be changed in the
Preferences dialog.