USN vs federal agency

USN and federal agency both are nouns.

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

As nouns, federal agency is a hypernym of USN; that is, federal 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
federal agency: an administrative unit of government

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

USN (noun)	federal 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