fitness vs suitability

fitness and suitability both are nouns.

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

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

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

Other hypernyms of fitness include suitableness.

fitness (noun)	suitability (noun)
the quality of being suitable	the quality of having the properties that are right for a specific purpose
the quality of being qualified
good physical condition; being in shape or in condition