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 – Flowing Intermediate Message Events
One of the 9 intermediate events in BPMN is the message intermediate event. There are two ways to use the message intermediate event, as an element in the sequence flow, or as an attachment to the boundary of an activity for exception processing. See how to use message intermediate events in a sequence flow. [Updated with correction of glaring error on Aug 22nd, 2006]