When you see something called rev. whatever, its usually a redesign of the current model and not an entirely new one. For example the Powermac, the graphite G4 would be rev. a, Quicksilver rev. b, Mirrored Drive Doors rev. c while the Powermac G5 would be a new model line so the current would be rev. a and the next would likely be rev. b. Sometimes rev. designations will be assigned to a slight variation on the model, like a speedbump and other minor things but it usually means an overhaul.
