compulsory, obligatory, required, mandatary, commandatory, ordained, imperatorial, commanding, orderly, official, more...

optional, voluntary, discretionary, elective, noncompulsory

required, compulsory, obligatory, prerequisite, precondition, more...

non, former, british, post, strong, minimum

not an option,   not optional,   no option,   must happen,   has to be done,   prescribed by law,   legally required,   have to be done,   it's not optional,   legally obligated