obligatory 10 Associations

bounden (similar term) compulsory (similar term) mandatory (similar term) required (similar term) de rigueur (similar term) imposed (similar term) incumbent on (similar term)

optional 8 Associations

elective (similar term) ex gratia (similar term) facultative (similar term) nonmandatory (similar term) nonobligatory (similar term) obligatory (antonym)