Apple Motion 3 - Manual (Page 292)

292 Chapter 3 Basic Compositing
With the enclosing group set to Pass Through, the Swirls object stencils all other
objects underneath it, including the Gradient object in the bottom group. The result is
that all objects are stenciled against the background color. The Fishes object is likewise
added to the combined stack of objects.
Normal
When a group is set to Normal, the layers nested within that group can only be
blended with one another. Objects nested within that group do not blend with objects
in other groups beneath it in the Layers list.
In this example, the Fishes and Swirls objects in the topmost group are only blended
with themselves when the topmost group is set to Normal.