9 vs digit

9 and digit both are nouns.

Word	Noun	Adjective	Verb	Adverb
9	Yes	Yes	No	No
digit	Yes	No	No	No

As nouns, digit is a hypernym of 9; that is, digit is a word with a broader meaning than 9:

Other hypernyms of 9 include figure.

9 (noun)	digit (noun)
the cardinal number that is the sum of eight and one	a finger or toe in human beings or corresponding body part in other vertebrates
	one of the elements that collectively form a system of numeration
	the length of breadth of a finger used as a linear measure

9 (adjective)	digit (adjective)
denoting a quantity consisting of one more than eight and one less than ten