RBAC differs from access control lists (ACLs), used in traditional discretionary access-control systems, in that RBAC systems assign permissions to specific operations with meaning in the organization, rather than to low-level data objects. For example, an access control list could be used for granting or denying write access to a particular system file, but it would not dictate how that file could be changed. In an RBAC-based system, an operation might be to 'create a credit account' transaction in a financial application or to 'populate a blood sugar level test' record in a medical application. The assignment of permission to perform a particular operation is meaningful, because the operations are granular with meaning within the application. RBAC has been shown to be particularly well suited to separation of duties (SoD) requirements, which ensure that two or more people must be involved in authorizing critical operations. Necessary and sufficient conditions for safety of SoD in RBAC have been analyzed. An underlying principle of SoD is that no individual should be able to effect a breach of security through dual privilege. By extension, no person may hold a role that exercises audit, control or review authority over another, concurrently held role.