central vs centrex

central and centrex both are nouns.

Word	Noun	Adjective	Verb	Adverb
central	Yes	Yes	No	No
centrex	Yes	No	No	No

As nouns, centrex is a hyponym of central; that is, centrex is a word with a more specific, narrower meaning than central:

central: a workplace that serves as a telecommunications facility where lines from telephones can be connected together to permit communication
centrex: (CENTRal EXchange) a kind of telephone exchange

Other hyponyms of central include patchboard, plugboard, switchboard.

central (noun)	centrex (noun)
a workplace that serves as a telecommunications facility where lines from telephones can be connected together to permit communication	(CENTRal EXchange) a kind of telephone exchange

central (adjective)	centrex (adjective)
in or near a center or constituting a center; the inner area
serving as an essential component