What is the primary purpose of a State Machine Diagram?

The primary purpose of a State Machine Diagram is to define object states and transitions. This type of diagram is essential in modeling the dynamic behavior of an object by capturing the various states it can be in throughout its lifecycle and detailing how it transitions from one state to another based on external events or internal conditions.

State Machine Diagrams effectively represent a system's behavior in a way that emphasizes the life cycle of an object, indicating the conditions that lead to state changes and the events that trigger these transitions. This information is vital for understanding how an object responds to different stimuli and for identifying any potential states the object might encounter during its operation.

Other choices focus on different aspects of modeling: for instance, illustrating class hierarchies pertains to representation in Class Diagrams, showing data flow relates to Activity Diagrams, and documenting user interactions would be more relevant in use case or interaction diagrams. Each of those diagrams has its own distinct purpose separate from that of the State Machine Diagram, underscoring the importance of using the appropriate diagram type depending on the aspect of the system being modeled.