Apple Motion 3 - Manual (Page 237)

Chapter 2 Creating and Managing Projects 237
Both arms and both legs are also made up of a hierarchy of nested body parts. It then
follows that these nested groups of layers, which connect to the torso, are themselves
nested inside the Mannequin group. When you collapse the arm and leg hierarchies,
you can see the following simplified hierarchy.
With all of these objects organized this way, animating the figure becomes easy. For
example, if you want to move the entire arm at once, you need only select the Left Arm
group, regardless of how many layers are nested within it.
Nesting groups to simplify a project
Selecting the Left Arm group selects all objects within it.