The word "nonmandatory" implies something that is optional or discretionary, but there are several antonyms that convey a sense of obligation or necessity. For instance, the word "mandatory" is the most direct antonym for this term, meaning that something is required, obligatory, or compulsory. Other related antonyms include "compulsory," "obligatory," "necessary," "essential," "compelling," and "imperative." These words suggest that an action, decision, or agreement is important, unavoidable, or necessary, without any room for choice or flexibility. In contrast, "nonmandatory" implies a degree of freedom or flexibility, and its antonyms convey the opposite meaning.