The National Institute of Justice provides guidelines, specifications, and documentation on specific threat levels that both soft body armor and ballistic plates are tested and certified under. The ballistic classifications for hard plates are:
Level IIIA Handgun protection up to .44 Magnum and 12 Gauge 1/2 oz. Slug threats
Level III Hard armor plates protection up to .308 M80 Ball threats.
Level III+ Ceramic Hard Armor protection up to 5.56mm M855 Green Tip special threats.
Level IV Ceramic Hard Armor protection up to .30-06 APM2 armor piercing threats.
The NIJ conducts tests and evaluations in their facilities for any manufacturers of body armor that pays and submits armor plates to the NIJ Laboratory for certification. They are subjected to round after round of gunfire from controlled projectile guns with recorded velocity, measured points of impact, and backface deformation. They also go through a rigorous environmental conditioning process involving extensive testing and evaluation by the NIJ. More information on ballistic protection levels can be found here.