Keying

Applies a lumakey. Anything below a certain threshold is transparent, and you can set a softness.

Allows you to record a clean plate and generates a difference matte. If you have a live feed you can capture the clean plate while the subject is out of shot, and once the subject is in shot it keys out the background allowing you to replace it with the underlying layer. Subject to the problems that difference mattes usually encounter (noise, lighting changes etc.)