A Use Case diagram shows a boundary box. One Actor is inside the box, and three are outside. What is the error?

Actors should never be inside the system boundary

Explanation

In a Use Case diagram, the System Boundary (the rectangle) represents the scope of the application or system being developed. Its primary purpose is to distinguish between what is part of the system and what is external to it.

Why Actors Must Stay Outside

• Definition of an Actor: By definition, an actor represents a role played by an external entity (a human, a device, or another system) that interacts with the subject.

• The Boundary's Role: Everything inside the box is a "Use Case" (a functional requirement or internal behavior). Since an actor is an external trigger or recipient of value, placing them inside implies they are a component of the system software itself, which is a modeling error.

• Primary vs. Secondary: Whether an actor is primary (initiates the process) or secondary (receives a notification), they both reside outside the boundary.