Scope of the Use Case Model

You will hear me say over and over again that it is absolutely critical that you keep the scope of each task foremost in your mind.
The scope of the use case model is to define how the users will interact with the system. Do not try to anticipate screen designs, system behavior, programming languages, or anything else that has to do with software. After all, if you do not first know what the system is supposed to do, then what is your basis for choosing between the implementation options?
There is a time and a place in the methodology for all the decisions required to develop a complete solution. Tackle one problem at a time, at the right time. Allow the products of one step to become a resource for subsequent steps. Following this approach supports the development of a repeatable and measurable process, key ingredients of a fully mature development methodology.

The Use Case Model describes the proposed functionality of the new system. A Use Case represents a discrete unit of interaction between a user (human or machine) and the system. A Use Case is a single unit of meaningful work; for example login to system, register with system and create order are all Use Cases. Each Use Case has a description which describes the functionality that will be built in the proposed system. A Use Case may 'include' another Use Case's functionality or 'extend' another Use Case with its own behaviour.
Use Cases are typically related to 'actors'. An actor is a human or machine entity that interacts with the system to perform meaningful work.