This page gives some basic details about the Coverage Render Element and how it is used in compositing.
The Coverage Render Element stores each pixels' final value taken from the object that makes the largest contribution to that pixel.
UI Path: ||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).