50 liters means 50 liters
This is a solid raincover; my only minor issue is that, on the Atmos AG 50, the hood of the pack cover slightly interferes with the load lifters. No serious functional impairment, just a bit awkward. But - this cover will fit a 50 liter pack, but no more. So, since Osprey pack volume can vary slightly depending on size (small, medium, or large), be sure you use the actual volume, not the number in the pack name, to select your cover. The salesperson who sold me my size large Atmos AG 50 thought the Medium pack cover (35-50L rating) would work. However, the size large is actually a 52 liter pack, and the medium would not fit a fully loaded size large pack (but would fit a small or medium.)
Glenn
Southwest Ohio