The DFD example here shows a context Data Flow Diagram that is drawn for a security trading platform. It contains a process (shape) that represents the system to model, in this case, the "securities trading platform". It also shows the participants who will interact with the system, called the external entities.
In this example, CS Assistant, Customer and Broker are the 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.
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