What does the large rectangular box in a use case diagram represent?

In a use case diagram, the large rectangular box represents the system boundary, which defines the scope of the system being modeled. This boundary helps to distinguish between what is part of the system and what lies outside it. Inside the box, all the use cases that the system supports are depicted, while outside the box are the external actors that interact with the system.

Understanding the system boundary is crucial for identifying the interactions between the system and external entities, which is fundamental in requirements gathering and defining how the system should function. It sets the context for the use cases, showing which functionalities are included within the system and which are outside its scope.

The other options do not accurately describe the function of the large rectangular box; they refer to different elements of system modeling but not to the boundary that encapsulates the system's functionalities.