Apple Motion 4 - Manual (Page 1172)

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.
• Subtract: Subtracts a mask from the alpha channel. This is useful for creating holes in
the middle of objects, or for masking out additional regions of an image that are
untouched by other masks. In the following example, both the rectangle and circle
masks are creating transparent regions.
1172 Chapter 17 Using Shapes, Masks, and Paint Strokes