A very useful feature is the option to use a black and white image or video as a luma matte for creating a new alpha channel.
Sometimes this technique is very useful because you can create mask by using a copy of the video in a black and white high contrast version which you create by using color filters.
On this approach is based the tracking matte system which is in Premiere Pro.

https://old.kdenlive.org/node/9458#AlphaChannel