It is not always necessary for the system to produce an output when it responds to an event. The distinction is crucial. Time to cancel a hotel room reservation which was not guaranteed for late arrival. Time to produce class rosters for instructors. An external event description is a complete sentence in the active voice.
Passenger requests an airline reservation.
Student drops a class. For example, in a university system Registration Request would be a good name for the input containing a student identifier and the list of desired sections. Similarly, to identify an actor means to Find out which people, organizations, or systems in the environment provide each system input or receive each output. Give the actor a name. The name of an actor is a noun or noun phrase describing the actor.