A subset of trusted systems ("Division B" and "Division A") implement mandatory access control (MAC) labels; as such, it is often assumed that they can be used for processing classified information. However, this is generally untrue. There are four modes in which one can operate a multilevel secure system: multilevel mode, compartmented mode, dedicated mode, and system-high mode. The National Computer Security Center's "Yellow Book" specifies that B3 and A1 systems can only be used for processing a strict subset of security labels, and only when operated according to a particularly strict configuration.