This page gives some basic details about the Coverage Render Element and how it is used in compositing.
The Coverage Render Element holds anti-aliasing data based on the Render ID Render Element. When the Coverage Render Element is multiplied by the Render ID Render Element in compositing, it adds anti-aliasing to the Render ID channel, providing a quick object mask.
||Render Settings window|| > Render Elements tab > Coverage
The parameters for this render element appear in the Attribute Editor under Extra V-Ray Attributes.
Enable – 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.coverage.vrimg).
This render element is not supported with V-Ray GPU rendering.