There is a distinction between 'objects' (which subclass MovableObject, and are movable, discrete objects in the world), and 'world geometry', which is large, generally static geometry. World geometry tends to influence the SceneManager organizational structure (e.g. lots of indoor static geometry might result in a spatial tree structure) and as such world geometry is generally tied to a given SceneManager implementation, whilst MovableObject instances can be used with any SceneManager. Subclasses are free to define world geometry however they please.
Multiple SceneManager instances can exist at one time, each one with a distinct scene. Which SceneManager is used to render a scene is dependent on the Camera, which will always call back the SceneManager which created it to render the scene.