Sheen is a useful view-dependent effect similar to a Fresnel effect in specular lobes of many materials.
Note that it is visible along the edges of objects as a reflective or shiny result. This is useful for creating looks like velvet or lustrous effects on stones and shiny fabrics. You may find this similar to using Edge Color alone in PxrSurface Specular effects.
This provides the resulting color of the effect either through a flat color, texture connection, or a procedural texture.
A greater value produces rougher or wider results that spread across the surface from the edge. At 1.0 it resembles a nearly diffuse surface and at 0.0 it's so thin it may be hard to see depending on the shape of your surface. Most objects will be realistic somewhere in between these values. Texturing this value may give you interesting effects like smudges, greasy fingerprints, and worn surfaces.
Sheen roughness examples : 0.0 / 0.2 / 0.4 / 0.6 / 0.8 / 1.0
Finger prints and smudges map applied to sheen roughness
Here is where you supply a signal, either a texture or procedural pattern, to create a bump to the surface to "fake" surface details like small bumps or scratches. This means an artist doesn't have to model these tedious and often repetitious parts of a model.
The above image is created with the following RenderMan network.
A Lamadiffuse is used to create the base material and a LamaSheen is layered over the top using a LamaLayer node. The LayerMix is lowered to 0.6 and the sheen is broken up with a fingerprint and smudge grunge map.