When an event initiated by external participant occurs (e.g. ordered goods are delivered by supplier), it isn't tied to the context (specific purchase order) yet. It means it's impossible to process such event withing a running instance of a process (we have more than one purchase order awaiting delivery). Therefore it should be processed by a dedicated event process (not to be confused with event subprocess).
The event process may be redundand if the process model is executable and the external participant is not a human being but a robot (automated system, web portal). In this case the message flow would model a web service call.
— Anatoly Belaychuk 2024-08-28 15:35