New AC reporting = Mitigation(AC)/Evasion(Avoiding Dmg)
AC stat reporting was overhauled by EQ awhile back. You'll find posts of SK's with like 15000 AC, but that is the old numbers.
Previously AC just added all the AC numbers from your armor/buffs/abilities. However tanks had noted for a long time that after certain AC totals,
you got much less benefit from adding more AC. It was the AC soft cap/hard cap. So you'd have 10K AC, but it would only be counting as 4k AC, and adding 300 AC only gave like 20 AC of actual benefit. Tanks used to run lots of parses to figure this out manually.
The new numbers are much more accurate so you can see the benefit of certain gear or buffs to really compare them. If you add a 300 AC buff, you'll see how much it helps. If you get a level of Combat Stability, you'll see it change your mitigation as it raises your mitigation soft cap. Same with Physical Enhancement (mit/evasion) and Combat Agility (evasion.) This is why tanks always list them as must have AA.
Shields bypass the AC cap, so you want the highest AC shield you can get with the highest AC aug you can get for the slot.
When I was first playing my SK, I was pulling my hair out trying to figure out how I was supposed to get to 15K AC when I could barely break 3k.