An article on CableOrganizer.com explains the details, and importance, of NEMA ratings for cabinets and enclosures that house networking and cabling equipment in unfriendly environments. NEMA, the National Electrical Manufacturers Association, established a rating system that is often used as a guide for identifying the extent to which cabinets and enclosures protect the products within them from environmental harm.
The article states, "NEMA-rated enclosures are all about controlling ingress and egress, a substance's ability to either enter or exit from a given structure or space - in this case, a cabinet. While the majority of NEMA ratings are mostly or entirely focused on blocking ingress, or the entry of a material into the enclosure, there are also several that deal with preventing egress, or the escape of a substance from the interior of an enclosure."
It then details the meanings of the following NEMA ratings: 1, 2, 3, 3R, 3S, 3X, 4, 5, 6, 6P, 12, 12K and 13.
You can read the full article here on the CableOrganizer.com site.
Complete specifications for NEMA enclosure ratings are also available here, on the NEMA site.