southwestern United States vs geographic region

southwestern United States and geographic region both are nouns.

Word	Noun	Adjective	Verb	Adverb
southwestern United States	Yes	No	No	No
geographic region	Yes	No	No	No

southwestern United States (noun)	geographic region (noun)
the southwestern region of the United States generally including New Mexico, Arizona, Texas, California, and sometimes Utah and Colorado	a demarcated area of the Earth