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 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 for more information.
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