Computer Security: (5) Access Control Models: (5. 4) Sensitivity Labels: In such a system subjects and objects must have labels assigned to them. A subject's sensitivity label specifies its level of trust. An object's sensitivity label specifies the level of trust required for access. In order to access a given object, the subject must have a sensitivity level equal to or higher than the requested object. (a) Data import and export: Controlling the import of information from other systems and export to other systems (including printers) is a critical function of these systems, which must ensure that sensitivity labels are properly maintained and implemented so that sensitive information is appropriately protected at all times. Two methods are commonly used for applying mandatory access control: (b) Rule-based (or label-based) access control: This type of control further defines specific conditions for access to a requested object. A Mandatory Access Control system implements a simple form of rule-based access control to determine whether access should be granted or denied by matching: (a) An object's sensitivity label; (b) A subject's sensitivity label; (c) Lattice-based access control: These can be used for complex access control decisions involving multiple objects and/or subjects. A lattice model is a mathematical structure that defines greatest lower-bound and least upper-bound values for a pair of elements, such as a subject and an object. Few systems implement MAC; XTS-400 and SELinux are examples of systems that do. The computer system at the company in the film Tron is an example from the prior century