A System Boundary element is a non-UML element used to define conceptual boundaries. You can use System Boundaries to help group logically related elements (from a visual perspective, not as part of the UML model).
In the OMG Unified Modeling Language specification, (v2.5.1), System Boundaries are described in the sections on Use Cases, because the System Boundary is often used to indicate the application of a Use Case to another entity. In this context, the System Boundary:
- Encloses the Use Case, and
- Is associated with a classifier such as a Class, Component or Subsystem (Actor) through the 'Select <Item>' dialog
By associating the System Boundary - and not the Use Case - with the classifier, the classifier is linked to the Use Case as a user, but not as an owner.
You can also define a Use Case as the classifier of a System Boundary element, to link the elements enclosed in the System Boundary (such as parts of an Activity diagram) to their representation in a logical Use Case.
The element properties for a System Boundary element comprise the name, the border style, and the number of horizontal or vertical swim lanes. You can also change the overall shape of the System Boundary, which includes an option to add dividing lines to the element other than by using the swimlanes, and you can make the element fully opaque, fully transparent or various degrees of opacity in between.
A System Boundary element can be marked as 'Selectable', using the element's context menu. When the element is not selectable, you can click on the other elements within the System Boundary space without activating or selecting the System Boundary itself.
- A System Boundary is the basis for the Image element, which enables you to add icons or backgrounds to a diagram, automatically displaying the Image Manager window from which to select the appropriate image
- A System Boundary is not the same as the Boundary element used to capture user interactions in, for example, Analysis diagrams
OMG UML Specification
The OMG Unified Modeling Language specification, (v2.5.1, p.641) states:
A subject for a set of UseCases (sometimes called a system boundary) may be shown as a rectangle with its name in the top-left corner, with the UseCase ellipses visually located inside this rectangle. The same modeled UseCase may be visually depicted as separate ellipses within multiple subject rectangles.