«إن كان ذلك الحكم لعلاقة بين المقدم والتالي سميت لزومية»
If that judgment arises from a relation between antecedent and consequent, the connective is called necessary (luzūmiyyah).
In a necessary connective, the two clauses are not merely side-by-side facts. A genuine relation makes the consequent relevant to the supposition of the antecedent. Such a relation can be conceptual, causal, grounded in a shared cause, or tied to correlatives.
“If this figure is a square, it has four sides” illustrates a conceptual link: four-sidedness is included in the concept of a square.