UML -- Use Case divisor (use case diagram)

I finally listened to a UML lesson seriously today. I feel that the teacher spoke well. Why do I think so? Of course, the most important thing is that what she said this time makes me not sleepy, but at the same time it also has my credit. Without my concentrated listening, I can't understand what the teacher said, to take a good course, the role of a teacher is indispensable. Although I have not learned much about these two sections, I will listen carefully in the future.

The following are some of the things I have learned today. I will write down my learned and learned knowledge in the future.
I. Functions of the use case diagram
It is mainly a language in which users interact with system analysts.
Ii. Composition of Use Cases
1: Participants: they can be people or things, as long as they are an instance that completes one thing and interacts with the system.
2: Use Case: the functions required by the system.
3: Relationship: There are three types of extension relationships (1): the selectivity of the Extension Unit (that is, whether the extension unit is to be executed depends on the user and other conditions)
(2) Inclusion relationship: shows the order of handling events (for example, collection is required before closing)
(3) Generalization: multiple methods for handling a certain event (for example, there can be two payment methods: cash payment and online bank payment)

Case model = case + case Specification Description