This article is from the Computer Security Evaluation FAQ, by Trusted Product Evaluation Program TPEP@dockmaster.ncsc.mil.
A class is the specific collection of requirements in the
Trusted Computer System Evaluation Criteria (TCSEC) to which an
evaluated system conforms. There are seven classes in the
TCSEC A1, B3, B2, B1, C2, C1, and D, in decreasing order of
features and assurances. Thus, a system evaluated at class B3
has more security features and/or greater confidence that the
security features work as intended than a system evaluated at
class B1. The requirements for a higher class are always a
superset of the lower class. Thus a B2 system meets every C2
functional requirement and has a higher level of assurance.