While I've seen the hard "rule" being written like Dru describes it, I think people also blur this quite a bit. For example, I've commonly seen stuff like "well, this would normally be a grade III route, but the horrible bushwack and opportunity for being caught in bad weather make it a IV or V" So I guess that brings in the question of if it's a commitment grade for the actual climb proper or just a commitment grade for avoiding an epic or getting caught out. If it's the latter, then I would definitely argue that something 3 days into the back country is a hellavu lot more commiting than something 3 minutes off the road.