A ModelNode represents a 3D model. The complexity of a model can vary from a simple MeshNode to a complex hierarchical tree of MeshNodes, LightNodes, and other types of SceneNodes. The ModelNode basically represents a "mini scene" stored under a single scene node. - ModelNode is the root node of this scene.

Loading and Cloning a Model:
A model can be loaded from an asset that has been preprocessed by the XNA content pipeline. For example:

ModelNode myModel = Content.Load<ModelNode>("dude");

The model node can be inserted in a IScene for rendering. The model node cannot be inserted multiple times into a scene. Also, it cannot be inserted into multiple scenes.

To render a model multiple times in a scene the model needs to be cloned by calling Clone:

// Clone the entire model.
ModelNode clonedModel = myModel.Clone();

Clone duplicates the ModelNode and its descendant nodes. The data objects referenced by the scene nodes, such as Mesh or Light, are not duplicated. The original model and the cloned model will references the same data objects. Only the instance information which is responsible for positioning the objects in the scene will be duplicated.)

Adding a Model to a scene:
To render a model, it needs to be added to a scene. ModelNode is derived from SceneNode, hence it can be attached to any node of a scene. For example, the following code inserts a model node under the root node of a scene.

// Insert model into scene.
myScene.Children.Add(myModel);

Reference