The figure below shows a context Data Flow Diagram that is drawn for a railway company's Customer Service System. It contains a process (shape) that represents the system to model, in this case, the "Customer Service System". It also shows the participants who will interact with the system, called the external entities.
In this example, Customer Service Assistant and Passenger are the two entities who will interact with the system. In between the process and the external entities, there are data flow (connectors) that indicate the existence of information exchange between the entities and the system.
Posted by: Manuel Kearney