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

In a use case diagram, the role of the actor encompasses both people and systems that interact with the subject of the diagram, typically a system or application. An actor represents any entity that has a goal in interaction with that system, which can include end users (such as customers or administrators) and external systems that communicate with the system being modeled.

By recognizing both human and system entities as actors, you can capture a more comprehensive view of the interactions and requirements necessary for the system's development. This holistic perspective is essential for understanding the various ways in which the system will be used and how it engages with external factors.

Other options might limit the scope of actors to only people or only use cases, which would not fully represent the breadth of potential interactions and could lead to incomplete requirement gathering. A complete understanding of all interacting entities helps ensure that all user needs and system interactions are considered in the analysis and design phases.