Bone layers are similar to Group layers - they can both group together multiple layers that
can then be manipulated as a single object. However, bone layers have an additional feature:
In a bone layer, you can set up a skeleton that can be used to manipulate your artwork. A
skeleton is exactly what it sounds like - an internal structure that can move the outer,
visible portion of your artwork.
There are two editing modes available for bone layers:
Bone, and
Animation.