If your project is large and uses a divide-and-conquer approach, such as decomposition, each subsystem would become a Subject when the work is at the subsystem level. From the perspective of each subsystem, however, any other communicating subsystem would be an Actor (an external source or sink of information, events, or data). Therefore, in one model, a model element may be a Subject, but in a different model, the same element may be an Actor, depending on what you are concentrating on for your System of Interest.