Masking and chromakey

I have a question about how masking and custom masks work within the DL2. Is there a place within the "global" section of the fixture to load custom masks or do you have to load them into the User Media and access them on one of the layers?

... and if you have to load them into the User Media, how can you get one to be a true mask via chromakeying without chromakeying every layer?... (or can that not happen?)