Subsystem   A subsystem is a grouping or package of model elements that has operations and interfaces. It has the characteristics of a package and a classifier. While a subsystem is a behavior element, it has no intrinsic behavior of it s own, the behavior is provided by the elements in the realization compartment of the subsystem.

Explanation

A subsystem is a way of grouping model elements to form a cohesive unit that represents both the behavior of the group of elements as a whole and also the way this behavior is realized by supporting model elements. It is really a type of package with the additional behavior of a classifier. Subsystems can be used to model products that have been incorporated into the system, or behavioral units in the system. An actor can communicate directly with a subsystem.

Important Points

		A subsystem can be divided into specification and realization compartments.		A subsystem contains other model elements, and these elements are contained in two compartments: a specification and a realization compartment. The specifications and the operations are realized by the elements contained in the realization compartment.
		A subsystem is both a package and a classifier, making it a powerful element , with both grouping and behavioral properties.		A subsystem is a grouping, and behavioral element that not only groups model elements but represents a behavioral unit in the system being modeled. It adds behavior, interfaces and operations to the package.
		Elements contained in a subsystem should not have public visibility.		The subsystem should not expose any of its workings; it should only be reached via its interfaces. Some confusion arises here, because a subsystem has compartments, which describe how the subsystem provides the behavior of its interfaces. This represents the difference between a modeler using the subsystem that can see into the subsystem and another model element that cannot see beyond the interfaces.

Related Entries

Package  Base Type A package is related to a subsystem in the sense that it is has the same fundamental shape. A subsystem is a type of package, but is also derived from a classifier, which gives the subsystem much richer semantics. The subsystem limits some of the visibility of contained elements, but while a package can define some of its contained elements to have public visibility a subsystem never does.