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A principal is the term we use to refer to the party for which (on whose behalf) an agent is executing an action. The Agent-Principal relation is an actor-party relation where the actor is executing an action on behalf of that party, and for the execution of which that party has established a policy that the agent must (or may) follow, and hence must have access to).
The Parties, Actors and Actions pattern provides an overview of how this concept fits in with related concepts.
The ability to distinguish between (non)principals is relevant in many situations, including:
- electronic communication: the principal
- A person that is 'doing its own things' is the Principal for himself (as an Actor); the person is also an Agent for himself (as a party).
- When a person is doing things for his employer, the latter is his Principal.