11. Use case diagram example

An use case diagram for an airline booking system is shown below

The top level diagram makes it simple to see what the airline reservation system consists of. It also shows who the actors are that interact with the system. The sales controller and ticket assistant are people, but another actor includes the airline accounting system.

The sub diagram for the Check in use case is also shown in more detail in the form of a sub diagram. You can see that the 'check in' use case includes the 'assign seat' case and the 'weigh baggage' case. Moreover, the 'weigh baggage' case itself has two special versions that deal with hand luggage and cargo baggage respectively.