The Billboard behavior node operates on the TransformGroup node
to cause the local +z axis of the TransformGroup to point at
the viewer's eye position.
The BoundingLeaf node defines a bounding region object that can be
referenced by other nodes to define a region of influence
(Fog and Light nodes), an application region (Background, Clip,
and Soundscape nodes), or a scheduling region (Sound and
Behavior nodes).
The BranchGroup serves as a pointer to the root of a
scene graph branch; BranchGroup objects are the only objects that
can be inserted into a Locale's set of objects.