Message Flow
A Message Flow is used to show the flow of messages between two entities that are prepared to send and receive them. In BPMN, two separate Pools in the Diagram will represent the two entities.
The Message Flow can connect directly to the boundary of a Pool, but only if the Pool does not have any process details within (e.g., is a “Black Box”).
A Message Flow can also cross the boundary of a Pool and connect to a Flow Object within that Pool.
If there is an Expanded Sub-Process in one of the Pools, then the message flow can be connected to either the boundary of the Sub-Process or to objects within the Sub-Process.