In UML models, subsystems are a type of stereotyped component that represent independent, behavioral units in a system. Subsystems are used in class, component, and use-case diagrams to represent large-scale components in the system that you are modeling.

You can model an entire system as a hierarchy of subsystems. You can also define the behavior that each subsystem represents by specifying interfaces to the subsystems and the operations that support the interfaces.

In diagrams, compartments display information about the attributes, operations, provided interfaces, required interfaces, realizations, and internal structure of the subsystem.

Typically, a subsystem has a name that describes its contents and role in the system.

As the following figure illustrates, a subsystem is displayed as a rectangle that contains the name of the subsystem. It also contains the keyword «Subsystem» and the subsystem icon.

A subsytem named Component1.