This channel controls the micro-surface information of the material, ranging from smooth to rough (maximum roughness being 1.0). In general, most materials are going to occupy the (.3-0.9) range of the roughness spectrum, with only exceptionally polished/smooth materials occupying a lower range. When using our provided Material Map Channel Packer tool, this channel also stores Specular AA information to make the effect free at runtime.
Couple examples, fairly short description
This is where the noise goes!
Comments about Specular AA should go here as well.