Apple Motion 4 - Manual (Page 1197)

Important: The children of 3D groups are composited in depth order by default.
Layers tab Objects sorted in depth order in Canvas
With the 3D groups above, objects are composited in depth order; their position in the
Layers tab does not correlate to their position in 3D space relative to the camera.
In the above example, the Blue A group is above the Red A group in the Layers tab but
it appears behind the Red A group in the Canvas because it is depth-sorted. The same
principle applies to the Blue B group and the Red B group.
1197 Chapter 18 3D Compositing