In one logical usage, ḥukm means affirming or denying a predicate of a subject. 'Prayer is obligatory' affirms a ruling of prayer; 'associating partners with Allah is not permissible' denies permissibility.
A declarative relation presents a proposition capable of being true or false. Understanding what a statement asserts is not the same as assenting to its truth. Hence judgment is not limited to sentences in the imperative mood.
When a proposition claims to state a religious ruling, its authority depends on appropriate evidence from revelation and the recognized methods of deriving rulings. Thus 'Zayd is present' and 'prayer is obligatory' can both be propositions, but they differ substantially in subject and evidentiary basis.