In principle RBAC supports the definition of arbitrary constraints on the different parts of an RBAC model.
However, at most efforts concerning RBAC constraints focused primarily on Separation of Duty constraints. With the increasing interest in RBAC in general and constraint based RBAC in particular, research pertaining to other types of RBAC constraints also gained in importance.
For example, a static separation of duties (SSD) constraint on two mutual exclusive Permissions prohibits an assignment of these permissions to the same Role.
Moreover, it also influences the definition of the respective role-hierarchy since it further prohibits that two distinct roles to which these permissions are assigned can have a common senior role. Otherwise a common senior could acquire both (mutual exclusive) permissions and thereby violate the corresponding SSD constraint. Similar effects can be observed for cardinality constraints for instance.
An example can be time constraints that restrict role activation to a specific time interval, or allow access operations for a particular resource only on a specific weekday.