The Selective toolset in the Image tools and Action is very powerful, allowing you to create a matte or alpha based on a key, gmask and 3D data. This matte or alpha is normally confined to the Selective due to the image processing pipeline.

But in this video, you'll learn about a new SelectiveFX shader known as "Selective Matte Extraction".

This allows you to push the matte or alpha from the Selective to the main output result of Action or the Image node.

You can take this output and use it further upstream in your flow graphs.