Interacting Subsystems in UML

In this paper we give a description of the subsystem construct in the Unified Modeling Language, emphasizing its dynamic aspects, thus giving a detailed description of the semantics of interaction with subsystems. Depending on whether the surroundings of the subsystem make use of public elements in the subsystem or not, the subsystem is considered to be open or closed, respectively. This leads to two different ways to use the services of the subsystem: either importing it and directly accessing its public elements, or associating it and only communicating with the subsystem itself. We also discuss some implications which closed subsystems have on collaborations