What is the function of a use case diagram?

A use case diagram serves primarily to provide a visual representation of the relationships between various use cases and the actors interacting with those use cases. It summarizes the overall functionality of a system from the user's perspective, capturing the essential interactions that users may have with the system. This overview helps stakeholders understand the scope and requirements of the system without delving into the program's underlying logic or architecture.

In contrast to options that detail program logic, class relationships, or system architecture, the essence of a use case diagram is to illustrate how users (or external entities) engage with the system through specific functionalities. It communicates what the system should do in response to user actions, emphasizing user-driven requirements and interactions rather than technical implementation. This summary is crucial in software development for ensuring that all stakeholder needs are considered and documented clearly.