A message flow describes the exchange of information between two pools. The message flow can be placed either directly between two pool objects, or between objects in the sequence flow of the processes in the corresponding pool. Only message flows are allowed to cross pool borders, and a message flow connection must not be placed between two objects of the same pool (see the figure Two pools with sequence and message flow).

The connection is represented by a dashed line. The beginning of the line is marked by a circle, and the end is a white arrow head.

Each message flow comprises a sender object, a connection of the sends type, a connection of the is received by type, and the recipient. No message flow connections may begin at a start event or intermediate event. However, an end event does not receive message flows, but can be a sender itself. Lanes, gateways, data objects, and text annotations do not have message flows.