That's what you should know about NIJ Standards!
The NIJ ballistic resistance standard classifies body armor by levels of ballistic performance. For any performance level, NIJ's test protocol requires that the bullet does not perforate the vest and that the vest protects against blunt trauma.
|NIJ Level IIA||NIJ Level II||NIJ Level IIIA||NIJ Level III||NIJ Level IV|
|Areal Density||3.5 kg/m||4.2 kg/m||5.9 kg/m||25.9 kg/m||32.5 kg/m|
|Thickness||4 mm||5 mm||6 mm||15 mm||20 mm|
|.30 Armor Piecing(M2 AP)||★|
NIJ's stab resistance body armor standard specifies the minimum performance requirements for body armor to protect the torso against slashes and stabs from knives and spikes; it also describes the associated testing procedures. The standard includes three performance levels, which are based on the armor's ability to prevent a perforation deep enough to injure an officer's internal organs at different strike force speeds. The standard also includes two protection classes: one for high-quality, commercially produced knives and another for lower-quality knife blades and improvised spikes that are likely to be present in a corrections environment.
National Institute of Justice, " Body Armor Performance Standards ," February 22, 2018, nij.ojp.gov: