NIJ Standards - Created by the National Institute of Justice
These standards outline exactly what each level should protect against. Anything from the small calibers like 9mm to .308 and beyond is accounted for in these standards. One thing to remember is that the order of most protection to least goes; IV, III, IIIA, II, and so on. The ones you'll see covered by standards are usually as low as II and as high as IV.
You maybe wondering what things like IV and III stand for. These are Roman numerals (Roman number system). IV = 4, III = 3, II = 2.
NIJ II stops up to .9mm
NIJ IIIA stops up to .44 Magnum
NIJ III stops up to 7.62 NATO
NIJ IV stops up to .308 AP ammo.
We didn't include all the different standard in this one. But, don't worry there are more articles to come where we will cover these topics more in-depth.