spelling vs US spelling

spelling and US spelling both are nouns.

Word	Noun	Adjective	Verb	Adverb
spelling	Yes	No	No	No
US spelling	Yes	No	No	No

As nouns, US spelling is a hyponym of spelling; that is, US spelling is a word with a more specific, narrower meaning than spelling:

spelling: forming words with letters according to the principles underlying accepted usage
US spelling: English orthography used in the United States (characterized by 'analyze', 'center', 'usable')

spelling (noun)	US spelling (noun)
forming words with letters according to the principles underlying accepted usage	English orthography used in the United States (characterized by 'analyze', 'center', 'usable')