Apple Motion 4 - Manual (Page 1236)

Receive Shadows: This checkbox controls whether or not neighboring objects’ shadows
will affect the current object. When deselected, light will affect the object as if the
shadow-casting object did not exist. The following image depicts an object (the white
ring) with the Receive Shadow checkbox selected and the Casts Shadow checkbox
deselected.
Shadows Only: This checkbox allows an object to block light and cast a shadow, while
the object itself does not appear in the scene.
1236 Chapter 18 3D Compositing