A system boundary defines what an analysis includes and what it treats as external. Boundaries can be conceptual, organizational, geographic, temporal, or institutional.
No model can include everything, so boundaries are necessary. They are also consequential: they shape which stakeholders, causes, costs, and interventions become visible. A boundary should therefore be justified by the decision purpose and tested against plausible alternatives.
Example
A delivery analysis limited to route time may exclude warehouse decisions, worker fatigue, customer constraints, traffic effects, and emissions. Expanding the boundary can change which intervention appears effective.