The entities can be classified into two types: "event" or "resource".
　A "event" type of entity must have an attribute of "DATE" while a "resource" cannot:
　order-date, delivery-date, invoice-date etc.

　An ordered pair consists of two objects considered in a particular order:
　the "event" entities are the ordered pair in "time sequence" (in historical order) with the time-stamp of "DATE".　Thus, the ordered pair (delivery, billing) is not the same as the ordered pair (billing, delivery).

　The "resource" entities are NOT the ordered pair:
　a tuple of (employee, division) and the (division, employee) put in reverse order have the same referential meaning, "assignment (or posting)--who is assigned to where." □