Keying

Keying is the term used to describe when you insert (or electronically cut) portions of one scene into another, or place titles over background images. Keys are made up of two basic components, an alpha, that cuts the hole in the background video, and a fill, that fills the hole with different video. Depending on the type of key, the alpha signal can be from an external source, or internally generated.

Keys, like MEs, are layered onto the background video from the lowest number key to the highest number key on the ME by default.