The data flow diagram example here shows a context Data Flow Diagram that is drawn for an Android supermarket app. It contains a process (shape) that represents the system to model, in this case, the "Supermarket App". It also shows the participants who will interact with the system, called the external entities.
In this example, there is only one external entity, which is the Customer. In between the process and the external entity, there is a bi-directional connector, which indicates the existence of information exchange between customer and the app, and the information flow is bi-directional.
