Actor: An actor is an external entity that interacts with the system, which can be a person, organization, external system, or hardware device, represented by a human-shaped icon.

Use Case: A use case is a service or functional unit of the system that an actor can perceive, describing how the system responds to requests from the actor, represented by a solid-line ellipse.

Relationship: The relationships between elements, including the relationships between actors, between actors and use cases, and between use cases.

Actors are external to the system, not part of the system;

Only those who use the system, interact with the system, or exchange information with the system are considered actors;

Actors are not necessarily human; they can also be other subsystems, other systems, time, temperature, or other factors.

A use case specification should include the following: the identification and name of the use case, the actors involved in the use case, a brief description of the use case, related other use cases, preconditions for executing the use case, basic event flow, alternative event flow, postconditions for executing the use case, and other information (such as non-functional requirements, design constraints, use case review status, author, modification records, etc.).

Between Actors: Mainly generalization, which is the relationship between general and specific.

Between Actors and Use Cases: Association represents the relationship between actors and use cases, indicating which actor can trigger which use case.

Between Use Cases: Relationships between use cases include generalization, inclusion, and extension.

Requirement Modeling: Use case diagrams are used to capture the functional requirements of the system, helping analysts identify the functional services the system should provide and how it interacts with external entities.

Functional Division and System Construction: By showing the logical relationships and invocation methods (such as inclusion, extension) between use cases, use case diagrams help system designers divide functional modules, establish structural hierarchies between modules, and support modular development of the system.

Role Identification and Permission Design: Use case diagrams define actors and their corresponding functions, helping the development team clarify the roles and permission scopes of various users in the system, providing a basis for subsequent permission control design.

Project Communication and Collaboration: Use case diagrams serve as a communication bridge between the development team, testers, customers, and other stakeholders.