Class IIa, II, and IIIa are what are considered "soft armor" designed to stop handgun and some subgun rounds ranging from 9mm to .44mag. Class III armor is the beginning of rifle armor and almost always uses some type of steel, or composite plate that is rated for multiple hits from .223, .308, and 7.62x39. Class IV Armor is rated for at least single-hit protection from 30-06 AP (M2). These plates are generally made of some type of ceramic and the dispersal of energy in stopping the AP round will shatter the ballistic plate hence the reason for only single hit protection.
This is a very brief overview and may contain slight errors in the info. It is what I remember from memory out of the [url=http://www.ammo-oracle.com]Ammo FAQ[/url] which also has a link to the official NIJ ratings on the net.
I've got my USMC Interceptor and my SAPI plates.... do you? But unless I have some way to conceal it I imagine a sniper would then aim for a head shot if he/she saw the armor.
Glad I'm on the West Coast... until it starts happening here God forbid. Stay safe over there you guys.
[edited cuz I'z dumm]