A Design Subsystem encapsulates behavior, providing explicit and formal interfaces, and does not (by convention) expose its internal contents. This provides the ability to completely encapsulate the interactions of a number of classes and/or subsystems.

 A Design Subsystem is a part of a system that encapsulates behavior, exposes a set of interfaces, and packages other model elements. From the outside, a subsystem is a single design model element that collaborates with other mod elelements to fulfill its responsibilities. The externally visible interfaces and their behavior is referred to as the subsystem specification. On the inside, a subsystem is a collection of model elements (design classes and other subsystems) that realize the interfaces and behavior of the subsystem specification. This is referred to as the subsystem realization.

The 'encapsulation' ability of design subsystems is contrasted by that of the Artifact: Design Package, which does not realize interfaces. Packages are used primarily for configuration management and model organization, where subsystems provide additional behavioral semantics.