This Use case data flow diagram depicts the interactions between the system you develop and the outside world. The actors represent these external users in the use case data flow diagram. Actual people, devices, hardware, or other systems that interact with the system could be used as actors. A use case represents the steps/actions that the actor will take to benefit from the interaction with your design in all cases. These steps can then be used to begin modeling how this will be represented in your system, typically as a class, sequence, or activity diagram at first.