it is implemented as the access control system in operating systems.

it is implemented as the access control system to provide Role-based Access Control (RBAC).

it is an abstract model (it is not implemented as it is)

it is implemented as the access control system in firewalls.

Role-based Access Control.

Capability lists.

Multi-level security.

Access control lists.

A user from level Secret can write documents in level Confidential.

A user from level Secret cannot write documents in level Top-secret.

A user from level Confidential cannot write documents in level Unclassified.

A user from level Confidential cannot read documents in level Unclassified.

the file is not executed because bob doesn't have write permission on the file.

the file is executed and its process has the permissions of user bob.

the file is executed and its process has the permissions of user alice.

the file is not executed because bob doesn't have execution permission on the file.

Define sets of mutually exclusive roles.

Define object hierarchies.

Define role hierarchies.

Define permission hierarchies.