VRay warning: "Render subsets and progressive sampling lead to artefacts along object edges."

[joelly3d]

I can't find anything on this warning. What exactly is a render subset? Scene has lots of VRay proxies and Rail clones, so finding the "thing" may prove to be difficult. Understanding the warning may steer me closer to a remedy.
Any insight appreciated.

[pixela]

I only encountered this message when ussing render mask in "selected" mode.
I think subsets mean the the objects you have selected.
I think it simply means the edges AA will not be correct, as it only renders the "selected" object.