Apple Motion 4 - Manual (Page 1237)

You can select both Receives Shadows and Shadows Only, which makes the object invisible
except for the regions where a shadow is cast upon it.
Shadows and Complex 3D Objects
When using shadows with objects such as particle systems, text objects, or replicators
that are set to 3D, shadows will be cast from one part of the object onto another part.
This is called self-shadowing.
1237 Chapter 18 3D Compositing