A Dependency (between two actors) indicates that one actor depends, for some reason, on the other in order to attain some goal, execute some plan, or deliver a resource. In a dependency between two actors, the former actor is called the depender, while the latter is called the dependee. The object around which the dependency centers is called dependum. In general, by depending on another actor for a dependum, an actor is able to achieve goals that it would otherwise be unable to achieve on its own, or not as easily, or not as well. At the same time, the depender becomes vulnerable. If the dependee fails to deliver the dependum, the depender would be adversely affected in its ability to achieve its goals. See Tropos: An Agent-Oriented Software Development Methodology (p. 207) for more information about this concept.