Apple Motion 4 - Manual (Page 265)

A group can also contain other groups nested inside it. In this way, you can construct
complex hierarchies of nested groups, with each nested group subordinate to the group
that contains it, up to the topmost group.
Nested groups
Enclosing group
One of the goals of organizing the layers in a project is to group layers that you want to
work together as a unit. Doing so ensures that animation and effects that are applied to
the enclosing group also affect all layers nested within that group. By nesting related
layers that you want to animate inside a group, you can save time by animating just the
enclosing group, instead of animating each layer individually.
For example, when you select a group that has three layers nested within it, the entire
group is selected as a single unit.
Layers list with group selected Group selected
265 Chapter 6 Creating and Managing Projects