A principal (of an actor) is a party on whose behalf the actor is executing an action. As, and in the context where the actor is doing this, we say that the party fulfills the role of principal for that actor. We also say that the actor fulfills the role of agent for that party.
Being a principal (or agent for that matter) is a role (characteristic/property) that a party (or an actor, respectively) only has at a point in time where the actor is actually executing an action on behalf of that party. At any point in time, a party can fulfill the role of principal for many actors, depending on the amount of actions that are being executed on its behalf.Principals are expected to provide its agents with the policies that provide the agents with the rules, working-instructions, preferences and other guidance that they need to comply with when executing the action. This, and relations with other concepts are described in the Parties, Actors and Actions pattern.
The purpose of being able to distinguish between parties that are, and that are not fulfilling the role of principal (of the actor that is executing some action), is that it is instrumental in discussions about who is (not) responsible/accountable for that action - including the way in which the action is executed, as well as the results produced by that action.
- When a person (as an actor) is doing something on its own behalf (as a party), it is both its own agent and its own principal.
- When a person does things for his employer the employer is that person's principal.
- when an ambassador acts in its capacity as ambassador, the country for which (s)he is ambassador is his/her principal.
- When the tax return form for a person or organization is filled in by some other person, the latter acts as an agent for that person or organization, which is then the principal.
- A company that makes cars is the principal of the robots that it uses to weld parts of a car together - but only when these robots are actually welding parts together.
- A retailer (organization) is the principal of a (running) webserver that accepts product orders.
- A wallet app that runs on a phone and that is exclusively used by a single person acts as a (digital) agent for that person, and the latter is its principal.
- When an enterprise hires an accountancy-firm to produce an accountants-report, and the latter assigns an accountant the task to actually do this, and if the accountant does so as an employee that
works forthe accountancy-firm (see the parties, actors and actions model), then the accountant acts as an agent and the accountancy-firm would be the principal. However, the accountant could also be 'outsourced' to the enterprise (in which case the enterprise would have onboarded the accountant) specifically for producing the accountants-report, then the accountant would be an agent for the enterprise, and the latter would be its principal.