poste restante vs social unit

poste restante and social unit both are nouns.

Word	Noun	Adjective	Verb	Adverb
poste restante	Yes	No	No	No
social unit	Yes	No	No	No

As nouns, social unit is a hypernym of poste restante; that is, social unit is a word with a broader meaning than poste restante:

poste restante: the part of a post office that handles mail for persons who call for it
social unit: an organization regarded as part of a larger social group

Other hypernyms of poste restante include unit.

poste restante (noun)	social unit (noun)
a notation written on mail that is to be held at the post office until called for (not in the United States or Canada)	an organization regarded as part of a larger social group
the part of a post office that handles mail for persons who call for it