UML learning-use case diagram

Use case diagram (Use Case daigrams)
The Use Case view of UML is the view of the external feature description of software products. It describes the demand for software products from the user's perspective rather than the developer's width, and analyzes the functions and dynamic behavior required by the product.
The use case diagram in UML describes a set of use cases, participants, and their relationships. Therefore, the use case diagram includes the following three aspects:
Use Case participant dependency, generalization, and Association
In the case view of UML, a participant (actor) is an entity (any thing or person that can be external to the system). It participates in the execution process of the case in some way.
A use case is a descriptive document used to describe the sequence of events that a participant (actor) uses to complete an event using the system.
Relationship between Use Cases
1. Generalization)
The generalization relationship between use cases is similar to that between classes. That is, in use case generalization, A subuse case represents a special form of the parent case. Child Cases inherit behaviors and attributes from the parent case. You can also add behaviors and attributes to change inherited behaviors.
2. Include)
The include relation separates the common steps of several use cases into a separate include use case. The inclusion relationship between use cases allows the behavior of the provider case to the event of the User case, And the inclusion case is called the customer case. The included use case is called the provider case, And the inclusion case provides the function to the customer case.
3. Extended relationship (extend)
Extension relationships are used to insert new behaviors into existing use cases. Basic use cases provide a set of extension points, where new actions can be added, while extended use cases provide a set of insert fragments, these fragments can be inserted into the extension points of the basic use case.