The mr Shader element outputs the raw contribution of any mental ray shader in the scene. The output of this element does not figure into the final rendered output.
For example, you could use the mr Shader element to render a few different ambient occlusion passes.
Note: The mr Shader element is intended primarily for rendering shaders, not full materials. While you can, in most cases, most "material-style" shaders might contain their own code to write to render elements, and in some circumstances this can cause odd conflicts. For example, piping a Standard material through a Material To Shader shader into the mr Shader element is not recommended because it could cause the Standard material’s render element to collide with the original render elements from the main render.
Shader elements can include standard 3ds Max materials and maps that are converted to mental ray shaders in the translation process.