The requirements of a system describe what the user expects from the system – in other words, what the system will do for the user. When defining the requirements of a system, the system should be viewed as a black box, so that only the external characteristics of the system are considered. Both functional and nonfunctional requirements need to be considered. Requirements modeling consists of requirements analysis and requirements specification.

Use case modeling is an approach for describing the functional requirements of the system, as described in this chapter. The system's data requirements in terms of the information that needs to be stored by the system are determined using static modeling, as described in Chapter 7. The inputs to the system and the outputs from the system are described initially in the use case models and then specified in more detail during static modeling.

This chapter gives an overview of software requirements analysis and specification in Section 6.1. It then goes on to describe the use case approach to defining functional requirements, as well as how to extend use cases to describe nonfunctional requirements. This chapter describes the concepts of actors and use cases, and then goes on to describe use case relationships, in particular, the include and extend relationships. Section 6.2 gives an overview of use case modeling followed by an example of a simple use case. Section 6.3 then describes actors and their role in use case modeling.