U.S. vs Southern United States

U.S. and Southern United States both are nouns.

Word	Noun	Adjective	Verb	Adverb
U.S.	Yes	No	No	No
Southern United States	Yes	No	No	No

U.S. (noun)	Southern United States (noun)
the executive and legislative and judicial branches of the federal government of the United States	the region of the United States lying to the south of the Mason-Dixon line
North American republic containing 50 states - 48 conterminous states in North America plus Alaska in northwest North America and the Hawaiian Islands in the Pacific Ocean; achieved independence in 1776