Note how in this diagram the relationships arrows point from the many back to the one. Thus an order has-a customer, and so has a field that links back to the customer for which this is an order. This is the diagram one would associated with a relational database model of the business objects - a customer cannot maintain a list of all its orders, so the orders are responsible for identifying the customer they belone to. In general a use case is a description of what a system is used for, and who uses it, without getting into details of how it it performs its functions. A use case focuses on the externally visible aspects of a system.
Use cases capture the functional requirements of the system that users use to perform their tasks.
Each type of user is called an actor . Actors can be people or other systems. Actors communicate with the system by sending and receiving messages . Use cases are explained and elaborated through scenarios . A scenario is a specific path through a use-case.