I'm working on a project to design a library management system using UML, and I'm stuck on whether to use a use case diagram or a sequence diagram for modeling the 'borrow a book' functionality. Can someone clarify the differences and when to use each?

Use case diagrams are ideal for capturing high-level functional requirements by showing actors and the use cases they interact with. They help in scoping the system and understanding what it should do without diving into details. For your library system, this is perfect for identifying key interactions like 'Borrow Book' at a macro level.

Sequence diagrams, on the other hand, focus on the chronological flow of messages between objects or components, making them excellent for detailed design and understanding specific interactions over time. In your case, once you have the use cases defined, a sequence diagram can illustrate the step-by-step process of borrowing a book, such as the member sending a request, the system checking availability, and updating records.

Best practice is to start with a use case diagram for overall functionality, then use sequence diagrams for complex or critical interactions to ensure clarity in implementation. This layered approach balances high-level planning with detailed design.