Apple Motion 3 - Manual (Page 1093)

Chapter 12 Using Shapes and Masks 1093
 Add: This is the default selection for new masks. Each new mask adds to the existing
alpha channel. This is useful for adding back regions of an image that other masks
are cutting out. In this example, both the rectangle and circle masks are creating
opaque regions.
If the layer already contains an alpha channel, a mask set to Add mode isolates only
the masked section of the original alpha channel.
Original alpha channel of butterfly clip Circle mask set to default. Add mode
applied to butterfly clip.