The subsystem contains components providing a common service. It has a boundary to which items can connect. You drag items inside the subsystem boundary, resize it to contain all of your required diagram items or group a set of related subsystems together with a frame box.

When you resize your subsystem, items retain their positions relative to the top and left boundary. As you drag the subsystem boundary two nested outlines represent your new subsystem, the outer one shows its perimeter, an inner one the position of contained items.

Subsystems can be the initial building blocks for State Diagrams and Activity Diagrams in the System Modes Model. You can link the subsystem to the diagram type, creating a new diagram which describes the subsystem.