Apple Motion 4 - Manual (Page 325)

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 tab.
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.
325 Chapter 7 Basic Compositing