I tend not to look at it in terms of 'lethality'. Taking the upper 10% (91-00) of a given crit column...
A = Disabling. You're not dying per se, but you can't contribute anything worthwhile to the situation until a healer has fixed you up. If you are conscious at all, doing anything besides screaming is a serious exercise in self discipline. Yes, that includes shutting up your screaming so as not to give your party's position away. Note that "not dying per se" doesn't mean you'll necessarily live without any medical attention at all. "You're taking _____ hits/rd" would fall into "not dying per se", but it'll still kill you if you don't do anything about it.
B = "Permanently" disabled. The damage may or may not kill you eventually if left untreated, but without expert treatment you're not only disabled for now, you'll be at least somewhat crippled for life. An actual medical expert can bring you back to 100% (eventually), but no amount of mere first aid/herbalism is going to get you back up to scratch. Shattered limbs fall into this category.
C = Crippled or killed. As 'B' above, only without some kind of treatment you will die from it eventually (maybe in a few rounds, maybe in a few days.) Without a medical expert, you not only won't ever be back to 100%, the affected area will be effectively useless. The "you'll never walk again" kind of crits. "Gut shot and broke your lower back" is a good example of this kind of crit.
D = Mortal wound. Without really serious medical attention, you have less than 2 minutes to live. Mere first aid may bring you up to 'stable', to the point where you have a day or two (in a coma) to get to a medical expert, but without that medical expert, you're gonna die, period.
E = Coup de Grace. You'll be dead in less than a minute, and likely as not you're dead before you hit the ground. Not only that, a medical expert is going to have a lot of work ahead of him getting your body where it will support life at all. On a solid 00, anyone with less than "nearly godlike" abilities as a healer doesn't have the ability to fix everything that's broken. Major central nervous system damage, destroyed heart and/or lungs, that sort of thing. "All that remains of foe are scattered bits of teeth and bone" is quite appropriate to my way of thinking. So is a crush crit that drives the sternum through the heart and into the spinal column.