There are various levels of armor protection. The higher the level number, the higher level of protection the armor is able to provide.
Without doing some research and reading the NIJ cert to see what type of penetration level six provides, suffice to say that level four is what most law enforcement agencies want for body armor.
Also, am on the road and don't have access to all my BORK info, so if someone else can look up what type of round/bullet is used for NIJ level six and pass it on, that would help Jellybeanie and others to understand how good this is. . .