If something is obligatory, you must do it because of a rule or law, etc.

The medical examination before you start work is obligatory. 工作前必须先体检。

[ + to infinitive ] The statute made it obligatory for all healthy males between 14 and 60 to work. 这条法令规定所有年龄在14至60岁的男子必须工作。

expected because it usually happens

Several Secret Service agents surrounded the President, all wearing the obligatory raincoat and hat. 几名特工围在总统四周，当然是一律穿着风雨衣，戴着帽子。