adj. 25B6; adjective COMPULSORY, mandatory, prescribed, required, demanded, statutory, enforced, binding, incumbent; requisite, necessary, imperative, unavoidable, inescapable, essential.
optional.
OBLIGATORY
Meaning of OBLIGATORY in English
Concise Oxford thesaurus English vocabulary. Краткий оксфордский словарь английского языка тезаурус. 2012