Body Armor Protection Levels Explained

Certification of Body Armor

The National Institute of Justice has a rating system for body armor.  As you add layers of a ballistic fiber, such as Aramid fiber, you add protection.
Vests are tested not just for stopping penetration, but also for blunt trauma protection – the blow suffered by the body from the bullet's impact on the vest. Blunt trauma is measured by the dent suffered by a soft clay backstop to the vest – a maximum of 1.7" (44 mm) is allowed.
NOTE: The standard NIJ test rounds are listed below – tested vests stop many other comparable rounds, and lesser threats.

NIJ 0101.05 Standard

Level	Tested for:	Comment
II-A	9 mm FMJ (Full Metal Jacket) at ~1,090 fps (332 mps) .40 S&W Full Metal Jacket at 1,025 fps (312 mps)	Minimum Recommended... ...for the vast majority of threats encountered on the street, though you would sustain more blunt trauma injury than Level II or III-A. The thinnest and best for concealability and comfort. Level II-A has fallen out of favor, so generally a special order item now.
II	9 mm FMJ, at ~1,175 fps (~358 mps) .357 JSP at ~ 1,400 fps (~427 mps).	A great balance... ...between blunt trauma protection, versus cost, and thickness / comfort / concealability. Handles the blunt trauma of higher velocity +P rounds better. What we recommend most often for concealable wear.
III-A	9 mm FMJ at ~1,400 fps (~427 mps) .44 Magnum Semi-Jacketed Hollow Point at ~1,400 fps (~427 mps).	The highest blunt trauma protectionrating in soft body armor. The best for very high-risk situations to cover more of the uncommon or unusual threats. Minimizes blunt trauma injury to allow more effective return fire.

It is important to know that Level II-A, II and III-A all stop the overwhelming majority of pistol projectiles you are ever likely to encounter (plus 12 gauge, OO buckshot), and also to know that NO armor is ever 100% ‘bulletproof’ under ALL conceivable circumstances.
However, these ratings often have a safety margin for penetration because blunt trauma is usually the limiting factor in certification. For example, Level II body armor would likely stop the III-A test standard, (9 mm submachine gun at 1400 fps) from actually PENETRATING through the Level II vest. But, the Level II vest would fail on blunt trauma impact (the NIJ deems any dent greater than ~1.7" (~44 mm.) on the soft clay test surface, a FAIL).
So, the advantage in increasing protection Levels from II-A, to II, to III-A, is NOT so much protection from PENETRATION of pistol fire, but a significant reduction in the blunt trauma received.

source