System boundaries are mainly used to show the scope of a system. It is important to be able to identify which use cases fall in our system and which don't. In a use case diagram, we only focus on the use cases in our system. In large systems, each module of the system is sometimes treated as a boundary if those modules are independent enough to function without each other. This is normally shown with a rectangular box that holds the use cases. An actor is not a part of the system, so the actor will be outside of the system boundary, as shown in the following diagram: