A message flow depicts the exchange of messages between two participants who are ready to send and receive them. A message flow must connect two distinct pools. They are either connected to the pool boundary or to flow objects within the pool boundary. They are not permitted to connect two objects in the same pool.
1] Silver, B. (2009). BPMN Method and Style: A levels-based methodology for BPM process modeling and improvement using BPMN 2.0. In CodyCassidy Press US.