You can layer shaders when you want to use more than one material for an object. Layered shaders let you create the appearance of variations in the material qualities of the surface by combining two or more material nodes that each have their own qualities. Layered shaders render more slowly than other materials, so consider using Layered textures instead to achieve similar results.
The Layered Shader node has an attribute called compositing flag that causes the shader type to layer materials or layer textures. You can use this to layer textures with the Layered Shader node. However, using the Layered Texture node is recommended because you can set many more options. See Layered textures for more information.
If you choose the texture option for the compositing flag, you must plug the whole network into a material (for example, Phong or Blinn).
For a description of this flag and other attributes of the node, see the Layered Shader node reference.
To create a layered shader (example)
The leftmost shader is the topmost material. You’ll have to have some level of transparency to see the shader underneath. So, for this example, you’ll need to map the Lambert’s transparency channel with something to see the shiny Phong underneath. Then assign the shader to an object.