Our economic system involves so many different transactions. An Observer conceives of a relationship between itself and a system. The nature of the relationship can be one of conceiving, selecting, ordering, etc. To the casual observer, the sun appears to orbit the earth. Reference establishes a standard from which an observer can infer associations of variables such as correlation or causality.
Reference creates a relationship for an observer.
External includes non-related relationship. There are many dangers external to the space shuttle's protective skin. A Whole is perceived by observers for its function on its own, instead for its specific relationships, subsystems, and parts. It quasi hides or has hidden its internal relationships from observation. Boundary collects all of the unobserved relationships in a system being observed, making wholes or entities.