Architecting and designing : Business Process Analysis (BPA) : BPMN : Modeling Sequence and Message Flows on business process diagrams
  
Modeling Sequence and Message Flows on business process diagrams
You can model the flow between events, processes, and artifacts in different pools by using Sequence Flows and Message Flows.
See also
Message Flows on business process diagrams
Drawing Sequence Flows on business process diagrams
Drawing Message Flows on business process diagrams
Definition of BPMN Message Flows
BPMN