Explanation

A model structured into smaller units is easier to understand. It is easier to show relationships among the model's main parts if you can express them in terms of packages. A package is either the top-level package of the model, or stereotyped as a use-case package. You can also let the customer decide how to structure the main parts of the model.

• If there are many use cases or actors, you can use use-case packages to further structure the use-case model. A use-case package contains a number of actors, use cases, their relationships, and other packages; thus, you can have multiple levels of use-case packages (packages within packages).
• The top-level package contains all top-level use-case packages, all top-level actors, and all top-level use cases.

Use

You can partition a use-case model into use-case packages for many reasons:

• You can use use-case packages to reflect order, configuration, or delivery units in the finished system.
• Allocation of resources and the competence of different development teams may require that the project be divided among different groups at different sites. Some use-case packages are suitable for a group, and some for one person, which makes packages a naturally efficient way to proceed with development. You must be sure, however, to define distinct responsibilities for each package so that development can be performed in parallel.
• You can use use-case packages to structure the use-case model in a way that reflects the user types. Many change requirements originate from users. Use-case packages ensure that changes from a particular user type will affect only the parts of the system that correspond to that user type.
• In some applications, certain information should be accessible to only a few people. Use-case packages let you preserve secrecy in areas where it is needed.