Bureau of Customs vs bureau

Bureau of Customs and bureau both are nouns.

Word	Noun	Adjective	Verb	Adverb
Bureau of Customs	Yes	No	No	No
bureau	Yes	No	No	No

As nouns, bureau is a hypernym of Bureau of Customs; that is, bureau is a word with a broader meaning than Bureau of Customs:

Bureau of Customs: the agency of the Treasury Department that enforces import tariffs
bureau: an administrative unit of government

Other hypernyms of Bureau of Customs include agency, authority, federal agency, government agency, office.

Bureau of Customs (noun)	bureau (noun)
the agency of the Treasury Department that enforces import tariffs	furniture with drawers for keeping clothes
	an administrative unit of government