The tracks V0, V1, V2, V3, and V4 are the video overlay tracks. The name stems from the fact that the contents of overlay tracks are pasted over the image obtained from the main tracks, VA and VB. The data from V0 are pasted first, followed by data from V1 and so on.

Video overlays are most useful when pasting a semi-transparent image, i.e. a video with an alpha channel. Most video clips don't contain the alpha channel, but you can easily prepare a semi-transparent bitmap file (or a sequence of bitmap files) and import it to the overlay track. The following example shows a logo pasted over the main clip:

The overlay clip needn't have the same dimensions as the background - it can be smaller or even larger, and it is always pasted (not resized!) so that the upper left corners of the background and the overlay coincide. This suggests a different use of overlay clips even if they aren't semi-transparent: You can take an overlay clip whose width is half those of the background to obtain two video clips joined in a single picture!

Audio overlays are probably not so useful as the video overlays; an audio overlay simply replaces the background audio. Again, the data from track A0 are used first, followed by the data from track A1 and so on.