This page gives detail about the Reflection Glossiness color channel.
Overview
The Reflection Glossiness channel stores the image's reflection glossiness as a grayscale image that represents the degree of Reflection Glossiness set for materials in the scene. White areas have the most reflection glossiness, while dark areas have little.
The amount of reflection glossiness for a material is set by its Reflection Glossiness parameter. For example, V-Ray Material has a Reflection Glossiness parameter, and Skin Material has parameters for primary and secondary reflection glossiness.
UI Path
||Node Editor|| > Add > Render Channels > Color Channel > Type > Reflection Glossiness
