Authorization Services provide Access Tokens to Clients on behalf of Users that they pass to a Server to be granted access to resources.
In a basic model (as shown in Figure 22) the Server is responsible for authorization (i.e. deciding what a user can do) while a separate identity provider (e.g. the operating system) is responsible for authentication (deciding who the user is).
In more complex models, the Server relies on external Authorization Services to provide some of its authorization requirements. These Authorization Services act in concert with an external identity provider which validates the user credentials before the external Authorization Service creates an Access Token that tells the Server what the user is a allowed to do. The Client interactions with these services may be indirect as shown in 6.2.2 or direct as shown in 6.2.3.
Even when the Server requires the Client to use an external Authorization Service the Server is still responsible for managing and enforcing the Permissions assigned to Nodes in its Address Space. The clauses below discuss the use of an external Authorization Service in more detail.