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A Boundary is a stereotyped Object that models some system boundary, typically a user interface screen. You can also create a Boundary as a stereotyped Class. Boundary elements are used in analysis to capture user interactions, screen flows and element interactions (or 'collaborations').
A Boundary is used in the conceptual phase to capture users interacting with the system at a screen level (or some other boundary interface type). It is often used in Sequence and Robustness (Analysis) diagrams. It is the View in the Model-View-Controller Pattern.