What is the role of the actor in a use case diagram?

The role of the actor in a use case diagram is to represent any entity that interacts with the system being analyzed. This can include both individuals, such as users or customers, as well as other systems that might exchange information with the main system. By defining actors this way, the diagram captures the full scope of interactions that can occur, allowing for a comprehensive understanding of the system's requirements and functionalities.

Including both people and systems as actors provides a more holistic view of the environment in which the system operates, facilitating better design and analysis. It emphasizes that interaction is not limited to human users but also encompasses other systems, promoting a broader understanding of how the primary system fits into a larger ecosystem.

The other options focus either solely on people or systems, which does not fully capture the diverse types of interactions present in many use cases. Understanding actors in this inclusive way helps stakeholders recognize all parties that influence system behavior and requirements.