text=A subsystem is a model element that contains a group of other model elements providing an independent, behavioral unit within a system. A subsystem comprises specification model elements and realization model elements. The specification model elements (for example, use cases and operations) provide an external view of the subsystem so that clients can interface with it. The realization model elements (for example, collaborations, classes, and nested subsystems) provide an implementation for the specified behaviors.
Typically, you use subsystems to decompose complex systems according to functional or implementation boundaries.