What type of rectangle represents an interaction use in a state machine diagram?

In a state machine diagram, an interaction use is represented by a rectangle with rounded corners. This specific notation indicates that the state machine is invoking a specific interaction or behavior that is defined elsewhere, such as in a use case or a sequence diagram. The use of rounded corners in the rectangle differs from the standard rectangle, which is commonly utilized to represent states and their transitions. By using rounded corners, it visually distinguishes interaction uses from other states and actions, thereby improving clarity in the diagram's representation of complex behavior.

This differentiation is crucial in understanding the diagram's context as users interpret the various components and how they integrate with other elements within the model. It helps in quickly recognizing parts of the system that are reusable or call upon defined interactions, which is a fundamental concept in systems modeling.