Obligatory (adjective)

1. Required by duty or rule; mandatory.
2. Expected or required as a matter of social custom or etiquette.

Origin:

From late latin obligatorius, from latin obligatus ("bound, oblige.d")

Examples:

1. Vaccinations are obligatory for all school children.
2. It's obligatory to wear a mask in the store.
3. The company made the training obligatory for all employees.
4. The meeting was obligatory for all members of the committee.
5. Sending a thank-you note is an obligatory etiquette after a dinner party.