Some explanations:
- "spec mix 1" and "spec mix 2" branches - 2-lobes glossy layer. It's used in modern skin shaders since at least 2013. It's also the reason why this shader is not based on PhysicalRoot. You need a good specular map and a good normal or bump map to really use advantages of this setup. Or use procedural enhancements or micropores map with usual quality maps.
- "Diff-SSS mix". Here you need a good SSS mask - not only for brows but for every place where bones are close to the surface. Also, a good mask would speed up the rendering process. Also, this mask is needed for make-ups, especially heavy or glittery make-ups, to block SSS on heavily painted zones.
- SSS radius. It's the simplest form of skin SSS profile. Starting values derived from a few published profiles. Why not a Poser native skin profiles? Because you cant' change these profiles, and every skin is unique. As far as I understand, included profiles are more or less generic caucasian profiles, so don't work very well both for darker skins and fore much paler skins.