I asked Ondra about the "2" few times, he told me it doesn't matter.
But honestly I still don't know about such feature...I do have scenes, where it's almost hard to identify whether interior or exterior has more content, while I want just interior to be sampled more.
I almost never have those clear cut "just air outside window, nothing else" scenes so I struggle how to place them efficiently, and unsurprisingly, it almost never does anything for me :- D