What are use class diagram, use case description and activity diagrams for use cases?

In Visual Studio, you can draw a use case diagram to summarize who uses your application or system, and what they can do with it. To create a UML use case diagram, on the Architecture menu, click New UML or Layer Diagram.For a video demonstration, see Organizing Features into Use Cases.To see which versions of Visual Studio support this feature, see Version support for architecture and modeling tools.With the help of a use case diagram, you can discuss and communicate:The scenarios in which your system or application interacts with people, organizations, or external systems.The goals that it helps those actors achieve.The scope of your system.A use case diagram does not show the detail of the use cases: it only summarizes some of the relationships between use cases, actors, and systems. In particular, the diagram does not show the order in which steps are performed to achieve the goals of each use case. You can describe those details in other diagrams and documents, which you can link to each use case. For more information, see Describing Use Cases in Detail in this topic.The descriptions you provide for use cases will use several terms related to the domain in which the system works, such as Sale, Menu, Customer, and so on. It is important to define these terms and their relationships clearly, and you can do that with the help of a UML Class Diagram. For more information, see UML Class Diagrams: Guidelines.Use cases deal only in the functional requirements for a system. Other requirements such as business rules, quality of service requirements, and implementation constraints must be represented separately. Architecture and internal details must also be described separately. For more information about how to define user requirements, see Model user requirements.The examples used in this topic relate to a Web site on which customers can order meals from local restaurants.Elements in a use case diagramAn actor (1) is a class of person, organization, device, or external software component that interacts with your system. Example actors are Customer, Restaurant, Temperature Sensor, Credit Card Authorizer.A use case (2) represents the actions that are performed by one or more actors in the pursuit of a particular goal. Example use cases are Order Meal, Update Menu, Process Payment.On a use case diagram, use cases are associated (3) with the actors that perform them.Your system (4) is whatever you are developing. It might be a small software component, whose actors are just other software components; or it might be a complete application; or it might be a large distributed suite of applications deployed over many computers and devices. Example subsystems are Meal Ordering Website, Meal Delivery Business, Website Version 2.A use case diagram can show which use cases are supported by your system or its subsystems.

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