Trusted Computer System Evaluation Criteria (TCSEC), the seminal work on the subject, provided the original definition of MAC as "a means of restricting access to objects based on the sensitivity (as represented by a label) of the information contained in the objects and the formal authorization (i.e., clearance) of subjects to access information of such sensitivity". Early implementations of MAC such as Honeywell's SCOMP, USAF SACDIN, NSA Blacker, and Boeing's MLS LAN focused on MLS to protect military-oriented security classification levels with robust enforcement. The term mandatory in MAC has acquired a special meaning derived from its use with military systems. In this context, MAC implies an extremely high degree of robustness that assures that the control mechanisms can resist any type of subversion, thereby enabling them to enforce access controls that are mandated by order of a government such as the Executive Order 12958 for US classified information. Enforcement is supposed to be more imperative than for commercial applications. This precludes enforcement by best-effort mechanisms; only mechanisms that can provide absolute or near-absolute enforcement of the mandate are acceptable for MAC. This is a tall order and sometimes assumed unrealistic by those unfamiliar with high assurance strategies, and very difficult for those who are. Strength Degrees In some systems, users have the authority to decide whether to grant access to any other user. To allow that, all users have clearances for all data. This is not necessarily true of an MLS system. If individuals or processes exist that may be denied access to any of the data in the system environment, then the system must be trusted to enforce MAC. Since there can be various levels of data classification and user clearances, this implies a quantified scale for robustness. For example, more robustness is indicated for system environments containing classified Top Secret information and uncleared users than for one with Secret information and users cleared to at least Confidential. To promote consistency and eliminate subjectivity in degrees of robustness, an extensive scientific analysis and risk assessment of the topic produced a landmark benchmark standardization quantifying security robustness capabilities of systems and mapping them to the degrees of trust warranted for various security environments. The result was documented in CSC-STD-004-85. Two relatively independent components of robustness were defined: Assurance Level and Functionality. Both were specified with a degree of precision that warranted significant confidence in certifications based on these criteria.