You have more than one pool in the process.
You have opened the Process Development perspective in the SAP NetWeaver Developer Studio and have expanded your project in Project Explorer view.
You create a message flow to show how messages flow between two separate process participants in the sense of separate business entities. In Business Process Modeling Notation (BPMN), two separate pools in the model represent these two participants. Message flows cannot be used between flow objects of the same pool.
Expand Process Modeling , then expand Processes .
In the context menu of a process, choose Open .
Expand Connections from Palette .
Choose Message Flow and choose a flow object in the modeling surface.
Not all flow objects in BPMN can be connected through message flows.
Create a message flow connection to another pool or a flow object in another pool using drag and drop.
To create a message flow connection to another pool, you must drag the connection to the other pool name area or to a flow object in the other pool. You cannot drag the connection to a lane, because the lane overlocks the pool.
(Optional) Choose General in the Properties view and specify a name of the message flow in the Name field.
(Optional) Specify a description of the message flow in the Documentation field.