USN vs government agency

USN and government agency both are nouns.

Word	Noun	Adjective	Verb	Adverb
USN	Yes	No	No	No
government agency	Yes	No	No	No

As nouns, government agency is a hypernym of USN; that is, government agency is a word with a broader meaning than USN:

USN: the navy of the United States of America; the agency that maintains and trains and equips combat-ready naval forces
government agency: an administrative unit of government

Other hypernyms of USN include agency, authority, bureau, federal agency, office.

USN (noun)	government agency (noun)
the navy of the United States of America; the agency that maintains and trains and equips combat-ready naval forces	an administrative unit of government