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The user view represents the goals and objectives of various users and their requirements from the system. The view represents that part of the system with which the user interacts.

System Users

System users can be human users or other systems that interact directly with the system. The role of a system user is defined based on the functions performed.
For example, consider the billing system of the department store from where you buy general groceries. The clerk who enters the product details will be the system user.

Actors

Actors are elements that lie outside the system and thus are called external entities of a system. They are used to model system users. Actors interact directly with the system, exchange messages, and request for some action to be performed. For example, in the billing system of the department stores, the clerk who enters the product details will be an actor.
External entities that do not interact directly with the system cannot be termed as actors of the system. For example, in the billing system of the department stores, as a customer, you will not be an actor, because you cannot interact with the system directly.

Use Cases

Use cases consist of elements that lie inside the system and are responsible for the working, that is, the functionality and behavior of the system. Therefore, use cases are nothing but the actions that the system performs to generate results requested by the actors of the system.
A use case is a set of transactions performed by the system when an actor initiates an event or action. Use cases represent all the possible events that can occur in all possible scenarios. They represent all interactions that can take place between an actor and the system.
A relationship represents a logical connection between use cases and actors. Relationships are unidirectional.
The following are examples of use cases in the billing system of the department store:
-   Enter Bill Details.
-   Calculate Total Amount.
-   Print Bill

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