NEW IN R21
Here you can link both BSDF layers you want to layer. BSDF layers can, for example, be output by the Uber Surface node.
This setting can be used to blend the result of layering BSDF 1 and BSDF 2. If set to 0%, the result of the shading description of BSDF 1 will be ouput; if set to 100%, the shading description of BSDF 2 will be output. In-between values will produce a corresponding mix of BSDF 1 and BSDF 2 as a result.