requisite, imperative, essential; see necessary 1.
"required." YourDictionary, n.d. Web. 05 February 2019. <https://thesaurus.yourdictionary.com/REQUIRED>.
required. (n.d.). Retrieved February 05th, 2019, from https://thesaurus.yourdictionary.com/REQUIRED
Incapable of being dispensed with:
Imposed on one by authority, command, or convention:
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