Exactly, depending on the make/model that's either 3.0V, 2.8V or 2.5V lowest discharge limit. If you go under that you basically can not longer trust the cell integrity and should dispose of it (no matter what miracle workers on the web say, the copper in the cell will have gone into solution and it can be deposited anywhere at all, but never again in the same spot where it was supposed to be so you are essentially the proud owner of an incendiary with a random length fuse...).

Ah, that makes sense. I guess it's kind of arbitrary (I mean, you could just call 80% full "100%" if that's all you'd use in practice) but it does seem quite useful if the "full" voltage is generally agreed to be in a narrow range and not have a wide range of maximum values depending on whether you care more about absolute capacity or longevity.