This is in the body of _run_event_driven_mission, so I understand event_id is the event ID indicated in the mission plan, meaning that when that asynchronous event is received, the execution of the corresponding sequences is triggered.

Now, DriverEvent is an enum with "events for driver state machines", so they are not events in the sense of those objects that are published somewhere and asynchronously received by subscribers. (Note that the same "event" term is used in the FSM context, but there they would probably be better called "commands" or "inputs" instead.)