Also user types are always used by an IMA Model. Actors can only serve as executing resource of a given task on a process if she or he or it belongs to a user type that is allowed to perform this task.
Further on, a resource can only choose its task preferences within the possibilities stated by the user type definition.

User Type Control defines all user types within the model.

User types can be defined and changed by means of User Type Control Services.
Also, the authorization (tasks to be executed, possible interfaces per task, etc.) can be defined for every user type.

The user type object model contains all user types. Per possible task the task authorizations are related to a user type.

The actors that can act conforming a user type are just passive objects in relation to a user type. Their control services are located in other servers.