fittingness vs suitability

fittingness and suitability both are nouns.

Word	Noun	Adjective	Verb	Adverb
fittingness	Yes	No	No	No
suitability	Yes	No	No	No

As nouns, suitability is a hypernym of fittingness; that is, suitability is a word with a broader meaning than fittingness:

fittingness: the quality of being suitable
suitability: the quality of having the properties that are right for a specific purpose

Other hypernyms of fittingness include suitableness.

fittingness (noun)	suitability (noun)
the quality of being suitable	the quality of having the properties that are right for a specific purpose