What does the use case diagram specifically illustrate?

The use case diagram specifically illustrates user interaction with the system. It is a visual representation that captures the functional requirements of a system by depicting various use cases, actors (users or other systems), and the relationships between them. Each use case represents a specific function or behavior that the system should perform from the user's perspective, highlighting how users will interact with the system to achieve their goals.

This focus on interaction allows stakeholders to understand the actions that users can take within the system, what functionalities the system provides, and how these elements are interconnected. It effectively communicates the user's needs and ensures that the system development aligns with user expectations.

In the context of other options, data storage typically relates to how and where data is saved within a system, rather than interactions. System architecture describes the overall structure of the system, including components and their relationships, rather than focusing on user interactions. Performance metrics are quantitative measures of system performance, which do not capture how users engage with the system.