Reference Shaders are an option under the Complex Shader in the Shader tree (only available when selecting from the top level of a shader) This allows you to create a shader tree that "subscribes" to existing shaders. 

This provides for consistency across multiple implementations of, in this case, a water and a foam shader. Each one uses the same reference for water and foam but you can change the scale and objects that the master shader (the multichannel  mixer) interacts with.