1.
If something is ~, you must do it because of a rule or a law.
Most women will be offered an ultrasound scan during pregnancy, although it’s not ~...
These rates do not include the charge for ~ medical consultations.
= compulsory
? optional
ADJ
2.
If you describe something as ~, you mean that it is done from habit or custom and not because the person involved has thought carefully about it or really means it.
She was wearing the ~ sweater and pearl necklace.
= customary
ADJ: ADJ n