Here is a brief summary of the protection against stab/knife threats offered by armor manufactured to each of the 3 NIJ ( USA National Institute Of Justice) categories. For a comprehensive document on protection levels and testing procedures, please download the NIJ Standard 0115.00 document from here.
Level I: This armor protects against low energy threats with a strike energy of 24J (17.7 ft. lbf). The overtest condition for this level is 36J (26.6 ft. lbf)
Level II: This armor protects against medium energy threats with a strike energy of 33J (24.3 ft. lbf). The overtest condition for this level is 50J (36.9 ft. lbf)
Level III: This armor protects against high energy threats with a strike energy of 43J (31.7 ft. lbf). The overtest condition for this level is 65J (47.9 ft. lbf)
Brief Technical Explanation:
For any given protection level, the test protocol requires the knife blade or spike to impact the armor test sample at two distinct energy levels. The first level, called "E1" corresponds to a condition where a maximum blade or spike penetration of 7mm (0.28 in) is allowable. The penetration limit was determined through research indicating that internal injuries to organs would be extremely unlikely at 7mm (0.28 in) of penetration. The test protocol then requires an overtest condition where the knife blade or spike kinetic energy is increased by 50%. At this higher energy condition, called "E2", a maximum blade or spike penetration of 20mm (0.79 in) is allowable. This overtest is required to ensure that there is an adequate margin of safety in armor design. Below is the summary of the strike energy protection requirement for the three protection levels: