Trace sets.

Trace sets is something that other renderers have, so sets that you set in either your shader or object so it can be included or excluded for for example SSS or ambient occlusion. At the moment in Blender if you set a ambient occlusion shader it will be occluded by either everything or nothing but nothing inbetween, with trace sets you can make it so it an be occluded by just selected objects.

Of course I'm sure that the Blender devs knows what this is, I want to make a feature request since there is none because I really think this is an essential thing for a renderer.