Apple Motion 4 - Manual (Page 340)

The order of two objects affected by the Exclusion blend mode does not matter.
Blend Modes That Manipulate Alpha Channels
The Stencil and Silhouette blend modes let you use a single object’s alpha channel or
luma values to isolate regions of background objects and groups.
Note: Similar effects can be accomplished using shape and image masks. In addition,
masks may provide you with a greater degree of control, depending on your needs. For
more information, see
Using Shapes, Masks, and Paint Strokes .
Stencil modes crop out all non-overlapping parts of objects underneath the object used
as the stencil. Silhouette modes do the opposite, punching holes in overlapping objects
underneath in the shape of the object used as the silhouette.
When working in a 3D group, changes in depth order affect the Stencil and Silhouette
blend modes differently. For example, if you have two layers in a 3D group and the upper
layer is set to Stencil Alpha or Stencil Luma, the blend mode remains in effect when the
upper layer is moved behind the lower layer in Z space. If you have two layers in a 3D
group and the upper layer is set to Silhouette Alpha or Silhouette Luma, the blend mode
does not remain in effect when the upper layer is moved behind the lower layer in Z
space.
340 Chapter 7 Basic Compositing