Apple Motion 5.1.1 - Manual (Page 917)

Chapter 21 3D compositing 917
Important: The children of 3D groups are composited in depth order by default.
With the 3D groups above, objects are composited in depth order; their position in the Layers list
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 list 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.
3D transformations in 2D groups
All objects have 3D transformations available, even when in 2D groups. Objects can be rotated
around any axis and moved along any axis. Objects in 2D groups are not depth-sorted, and
cannot intersect, regardless of their position in 3D space.
In the image below, groups A and B are positioned at the same point in 3D space. But because
they are layer-ordered, group A does not intersect with group B.
However, when you change the parent group to 3D, groups A and B intersect, as shown in the
image below.
67% resize factor