When you look into the Sandy3D 3.0 documentation, you may have seen a classe : Scene3D. But what is it?

A Scene3D object is the entry point of any Sandy3D application.
It has a container to render the scene, a camera and a root node.
So the process is:

1. render the scene from the root node.
2. project everything from the given camera
3. display the result inside the container

World3D uses a Singleton pattern. World3D is in fact useful when using a single scene.
It is simpler to use than Scene3D, and compatible with previous versions of Sandy.
But a World3D object is in fact a Scene3D object behind the scenes (sorry for the pun).