The Edit Page : Editing Your Project : Compositing

Compositing
To start your own composite, drag a clip from the Library directly on top of a clip on the Storyboard. Two options appear:
Replace
- You can choose “Keep length” which does not change the length of the clip on the Timeline, or “Use new length”. If you choose “Keep length” and the new clip is shorter, a black clip will be added for the remaining duration of the original clip. This option will maintain sync across all tracks of your timeline.
Composite
- If you choose this option, the new clip is placed as a picture-n-picture over the existing clip. Tap on the picture-in-picture in the Preview to make adjustments.