Compositing and Masks
Compositing is the process of mixing video tracks to create a single layered output.
You can use compositing in conjunction with masks to cover portions of video or to limit the effects of a filter. Masks are frequently based on dark versus light areas, specific colors, or an alpha channel.
Click the Parent Composite Mode or Compositing Mode button in the track header and choose a mode from the menu to determine how the transparency in a video track is generated. Since lower tracks show through higher tracks, it is the compositing mode of the higher track that determines how much of the lower track shows though. The compositing mode of the lowest video track adjusts its transparency against the background.