Protection Classes
German and American protection classes are not interchangeable, even though calibers used for shooting tests are similar.
General conditions differ significantly.
American test pieces are only shot at once, while German ones are shot at three times.
Environmental test conditions are also different.

Nevertheless, the German protection class (SK) 1 is generally equated with the American NIJ-Level IIIA. Both protection classes are defined by protection from current pistol calibers which are generally similar.

The German SK4 and American NIJ-Level III and IV are also often compared with each other. These classes define protection from rifle fire.

Protection classes can be summarised generally:
SK L: Protection from pistol bullets with soft core and round nose

SK 1: Protection from pistol bullets with soft core and round nose

SK 2: Protection from current pistol ammunition including hard core material

SK 3: Protection from rifle ammunition with full metal jacket and soft core

SK 4: Protection from rifle ammunition with full metal jacket and hard core material

Thrusting weapons, e.g. knives or needles, are not necessarily included in the first two protection classes and must be provided in addition. This is called puncture protection. For protective vests to meet German protection class SK 3 or 4 standards they must be constructed to offer puncture protection.

Abbreviations:
FMJ, RN = full metal jacket, round nose
SC = soft core,
HC = hard core,
JSP = jacketed soft point