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 UML Superstructure Specification, v2.1.1, 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 Sub-system (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.
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 dialog 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|
|•||Mapping Use Cases to Activity Diagrams (Online resource)|
OMG UML Specification
The OMG UML specification (UML Superstructure Specification, v2.1.1, p. 594) states:
If a subject (or system boundary) is displayed, the Use Case ellipse is visually located inside the system boundary rectangle. Note that this does not necessarily mean that the subject classifier owns the contained Use Cases, but merely that the Use Case applies to that classifier.