Use Case: Patterns and blueprints (Use Case: mode and blueprint)

Use Cases can capture the funtion requirements of your systems and definate the border of the system.
One use case can simulate one of the using ways of your system, that is, the event that triggered by users from outer system what sequence actions will be adpted.
Each use case is a complete system usage method. Includes the description of internal events caused by user input and system response to these inputs, as well as internal computing and output to users. Use Case (scenario) : Describe an execution path of the use case with actual values.
Each use case has a purpose and provides a value for the stakeholders. The Use Case name is From a system Perspective A verb phrase given in the active current tense. It usually contains several words.
Sometimes it is helpful to classify use cases based on their importance to the system. Main Use Cases That is, if it is withdrawn from the system, it will be meaningless. Secondary use case Supports the performance of main use cases. Optional. It may be included in a certain installation of the system, but not included in other installations.
Gray box view of the system
Use Cases are independent from the internal structure of the system. The use case describes what happened inside the system, but does not display the internal structure of the system, and does not include actions that depend on the internal structure of the system.
It is impossible for one use case to call another. The sequence of actions executed by one use case can be combined with that of another use case. However, use cases can indirectly affect each other by using the same information in the system.
The Use Case model must capture each sequence that may occur during system use. Organize Use Cases: bind all variants of the same topic, that is, variants, exceptions, and error variants of the same basic action sequence to a unit.