Intermediate message events can be placed on an activity boundary in BPMN. This use represents a form of exception handling for a task or sub-process, in response to an incoming message. This is a very different behavior than the ones we previously described, which show how to use intermediate message events in the course of the normal sequence flow of a business process.
BPMN Diagrams – How To Use Subprocesses
This article shows how to use the different types of subprocesses in BPMN diagrams. Examples of normal subprocesses, looping, multiple instance, ad hoc, and compensation subprocesses.