central office vs plural

central office and plural both are nouns.

Word	Noun	Adjective	Verb	Adverb
central office	Yes	No	No	No
plural	Yes	Yes	No	No

central office (noun)	plural (noun)
(usually plural) the office that serves as the administrative center of an enterprise	the form of a word that is used to denote more than one

central office (adjective)	plural (adjective)
	grammatical number category referring to two or more items or units
	composed of more than one member, set, or kind