Apple Motion 3 - Manual (Page 1094)

1094 Chapter 12 Using Shapes and Masks
 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 this example, both the rectangle and circle masks are
creating transparent regions.
 Replace: This setting uses the current mask to completely replace the object’s
original alpha channel, as well as any other masks that are applied to the same
object that appear underneath the current mask in the Layers list. You can add
additional masks above, set to whatever blending mode you like. In the following
example, the circle mask is overriding the rectangle, since it is at the top of the masks
that are nested within the gradient object.