The eight forms differ along two connected dimensions: the force of the claim and the period or condition for which it holds.
Necessity: the contrary cannot obtain under the stated qualification.
Permanence: the relation continues throughout the specified period; permanence alone does not automatically establish necessity.
Description: a condition linked to the subject's descriptive title is maintained.
Time: necessity is attached to a specified or unspecified time.
Actuality and possibility: one may assert actual occurrence or deny the necessity of its opposite.
The most serious confusions are inferring “necessary” from “has happened,” or “actual” from “possible.”