What does a system boundary represent in use case diagrams?

A system boundary in use case diagrams is represented by a shaded rectangle. This rectangle delineates the scope of the system being modeled, distinguishing what is inside the system from the external environment. It helps to clarify which functionalities are part of the system and which are outside it, thus providing a clear visual representation of the system's limits.

The boundary serves a crucial role in identifying the actors that interact with the system, highlighting their roles and the specific use cases associated with them. By defining these boundaries, stakeholders can better understand the intended functionalities, interactions, and constraints of the system in question. This visual component is vital for effective communication among developers, stakeholders, and users, ensuring everyone has a common understanding of the system's scope and interactions.