This page gives some basic details about the Reflection Glossiness render element and how it is used in compositing.
The Reflection Glossiness Render Element 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 (VRayMtl) has a Reflection Glossiness parameter, and Skin Material (VRaySkinMtl) has parameters for primary and secondary reflection glossiness.
||Render Settings window|| > Render Elements tab > Reflection glossiness
The parameters for this render element appear in the Attribute Editor under Extra V-Ray Attributes.
Enabled – When checked, the render element appears in the V-Ray Virtual Frame Buffer.
Deep output – Specifies whether to include this render element in deep images.
Filename suffix – The text added to the end of the rendered file, when saved as a separate file (e.g. myrender.reflGloss.vrimg).
This render element is not supported with V-Ray GPU rendering.