An actor consists of an entity that has strategic goals and intentionality within the system or the organizational setting. An actor represents a physical, social or software agent as well as a role or position. While assuming the classical AI definition of Software Agent, that is, a software having properties such as autonomy, social ability, reactivity and  proactivity, Tropos defines a role as an abstract characterization of the behavior of a social actor within some specialized context or domain of endeavor. A position represents a set of roles, typically played by one agent. An agent can occupy a position, while a position is said to cover a role. See Tropos: An Agent-Oriented Software Development Methodology (p. 206)  for more information about this concept.