a nutshell, NIJ 06 Certified Rifle Plates typically have 0.25"
(6 mm) of foam padding on the front (strike face). This is to better
withstand damage from the impact of the tough drop test, where the plate
is slammed down on a steel deck, and then X-rayed to check for cracking.
if you think you might be going down hard on your chest, for example,
NIJ 06 plates are less likely to be damaged by the impact. The best
choice if rough use is expected.
tradeoff is that NIJ 06 plates are ~0.25" thicker, weigh a little
more, and cost a little more than the traditional NIJ 05 Standard plates.
So for less rough duty, the NIJ 05 plates may work for you.