A Message Flow shows the flow of messages between two model elements that are prepared to send and receive them. A model element in this case is either an organizational unit, a function, an application, a class, an entity, or a location.

The model elements is represented on the BPMN Business Process diagram by a pool or swimlane. To show the message flow between model elements, you can draw a Message Flow line between the pools, or between the processes within a pool.

One advantage that the BPMN Business Process diagram offers over traditional business process diagrams is the ability to model message flows. Traditional business process diagrams enable the modeling of sequential process flows – from starting events to ending results. In an eCommerce environment, however, people send messages to one another as part of the process flow. These messages drive the depiction and understanding of business-to-business and business-to-consumer processes. The BPMN Business Process diagram enables you to model message passing using the Message Flow line.

Messages are sent between two different departments or organizations. This rule is enforced by mandating that a Message Flow line be drawn between two processes in different pools. They cannot connect two processes within the same pool, even between two lanes of that pool. If you try to connect a Message Flow line between two processes in the same pool, lane, or across lanes in the same pool, System Architect does not allow the connection.

A Message Flow line can be drawn to a process within a pool or lane, or to the edge of the pool or lane itself.

You cannot draw a Message Flow to or from a Gateway.

You can fill in or observe the following properties in the definition dialog of a Message Flow.