Nodes and Groups This chapter explains the concept of a scene graph, and shows how to build scenes out of groups, properties and shapes. It also describes the concepts of actions, traversal, and traversal state.
Programmers can extend the toolkit to support almost anything.
I've also gone a little beyond just summarizing the book here, adding my own comments as notes. Each node implements its own action behavior. Each field class defines methods to get and set its values. The way shapes are drawn is affected by property nodes in the scene. A property must be drawn before a shape node to affect it.