System boundary
The set of processes, sources and activities included in an assessment.
Example
A product study may include raw materials, manufacture, use and disposal.
The catch
Moving the boundary can change the result without changing physical emissions.
In more depth
System boundary, explained.
A system boundary should be drawn before the results are known, because a boundary chosen afterwards can be shaped to produce a flattering number. In a life-cycle assessment, cut-off rules let analysts exclude flows below a stated significance threshold, which is reasonable when the threshold is declared and tested.
In a project, the boundary must include emissions the project itself creates, such as electricity for blowers or fuel for trucks, and effects that spill beyond it are handled as leakage. A good boundary statement comes with a diagram, a list of exclusions and an estimate of what each exclusion is worth. Compare results only when their boundaries match, including the gases covered.