Class A : regular buffs/debuffs/skill
Class B : elemental % and pets damage %
Class C : elitist affixes
Damage dealt to elites/beasts/demons/humans.. are increased by x%
Class D : BoT
Bane of the trapped - you deal x% more towards cc'ed enemies.
Class E : Zei's
Zei Stone of Vengeance - you deal x% more towards enemies y yards away
Class F: Some passive boosts
Some classes have extra passives that stack here, like the Monk's Mythic Rhythm or the Wizard's Audacity. Not sure about barb.
Class G: Some spells
Some classes have spells that go here instead of in class A, like the Wizard's Hydra. Not sure about barb.
Class H and beyond:
Anything else that is in its own class. I've never found a comprehensive list.