UML-use case diagram

Is a simple use case chart The Use Case chart is mainly composed of a villain, an elliptical person, and an arrow. It looks pretty nice. So why is it used to pinch the villain and an elliptical person? What does it mean? After reading it, you will know it.
Use case diagram is used to describe what a role can do through the system. use case diagram focuses on the external performance of the system. The interaction between the system and the human interaction system and other systems

Villain: By classifying users in the system, you can extract different roles, different roles, and different responsibilities. Different system functions are used. Every villain is a role. It gives us a better understanding of the problem from the perspective of different roles. For example, to develop a data center charging system, it is easy to think that the permissions of the three roles are obviously different for the user operator Administrator. the ellipse presents these functions. PS: roles can be human or other systems
Elliptic: In addition to the following dynamic object phrase, the elliptic form a use case to indicate what the system can do. The data center charging system can also be used as an example to recharge and drop down PS: one use case can belong to multiple roles
Arrow: Arrows indicate that a role can be used as a use case. Arrows indicate that the data flow is directed to the use case. If the role inputs data to the system and points to the role, the system outputs data to the role. PS: I have seen a line without arrows in other places, indicating that there is no clear data flow. This is not very well understood.