In Mixer, you can directly control a layer’s mask: a greyscale image, which controls the visibility of a specific layer. At the bottom of the Layer panel you’ll find buttons enabling procedural workflows for even more control over your Mix. This section contains information on each type of mask available in Mixer.
You can also add a Mask Stack to the layer by right clicking on a layer and selecting Add Mask Stack from the options available or by clicking the mask stack icon at the bottom of the
You can also add a mask on the layer by selecting the desired layer and then clicking the mask icons at the bottom of the layer stack.
Here are the three types of masks supported by Mixer.