requisite, imperative, essential; see necessary 1.
"required." YourDictionary, n.d. Web. 15 October 2018. <http://thesaurus.yourdictionary.com/Required>.
required. (n.d.). Retrieved October 15th, 2018, from http://thesaurus.yourdictionary.com/Required
Incapable of being dispensed with:
Imposed on one by authority, command, or convention:
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