From The Firearm Blog:

Most people will care more about 0123.00 as they are shopping for new armor. There are five types of armor rating. HG (for handgun) 1 and 2, and RF (for rifle) 1, 2, and 3. First up we have the handgun levels. The old system was, in order of increasing protection, IIA, II, and IIIA, which makes almost no sense. Moving to only two ratings makes sense because IIA is fairly uncommon these days.

There are only two handgun protection levels under NIJ 0123.00. HG1 handles 9mm and .357 magnum threats. HG2 also covers 9mm and adds .44 magnum. Of note, the test spec for the 9mm load for HG1 is 1,305 feet per second. For HG2 the specified velocity is 1,430 feet per second. That is a sout load with 124-grain 9mm ammo. Both of those loads are running quite a bit faster than the 1,000-1,100 FPS that 124-grain ammo will do from the average handgun, and look more like submachinegun (HG1) and 16″+ PCC (HG2) stats. The magnum revolver loads both clock in at 1,430 feet per second.