When you hit a single mob or player with a spell, or some kind of crude inertia-based impact utensil, the game will work out how much damage that target takes. This is based on the various offensive properties of you and the spell or utensil, as well as the defensive properties of your target. The same is true for area-of-effect (or AoE) abilities, though these tend to do less damage to a single target. Add in some more targets and, while it's still fun to do lots of damage to one of them (with the casting and the poking with sticks), you may have a chance to do damage to all of them at once. Let's say that you can do 2500 damage to a single target with one spell or stab, but can only do 1000 damage to a single target with your AoE ability. If you have five targets that you can hit with your AoE, then that will do a total of 5000 damage. Already we're having more fun than just beating on the one target.
So we're doing 1000 damage to every target and as we keep adding targets we keep doing more total damage. The current AoE damage cap is on that total amount of damage. Once we reach that total then we simply can't do any more. If more targets are added then the amount we do to each is reduced, to keep our total on the cap limit. The change in Patch 3.3 PTR doesn't have a value for the total damage you can do. Instead it works out how much damage you would do to ten targets and limits you to that. This means that your AoE spells will do the normal amount of damage to a group of ten or less targets but as soon as you add an eleventh (or more) your damage per target is reduced. To quote the patch notes:
In other words, if a spell does 1000 damage to each target, it would hit up to 10 targets for 1000 each, but with more than 10 targets, each target would take 1000 damage divided by the number of targets. 20 targets would be hit for 500 damage each in that example.Depending on the current AoE damage cap this change might mean we do more or less damage to a large group of targets (probably less). What it does mean is that the damage cap is more predictable, and more scalable. We'll be able to know at what point our AoE abilities start losing power by counting the targets and this will adjust as we increase in level and improve our gear.
Patch 3.3 is the last major patch of Wrath of the Lich King. With the new Icecrown Citadel 5-man dungeons and 10/25-man raid arriving soon, patch 3.3 will deal the final blow to the Arthas. WoW.com's Guide to Patch 3.3 will keep you updated with all the latest patch news.