add-on vs fitting

add-on and fitting both are nouns.

Word	Noun	Adjective	Verb	Adverb
add-on	Yes	No	No	No
fitting	Yes	Yes	No	No

As nouns, fitting is a hyponym of add-on; that is, fitting is a word with a more specific, narrower meaning than add-on:

Other hyponyms of add-on include auto accessory, computer accessory, fixings, trimmings.

add-on (noun)	fitting (noun)
a component that is added to something to improve it	putting clothes on to see whether they fit
a supplementary component that improves capability	a small and often standardized accessory to a larger system
a software module that extends or enhances the capabilities of an existing application	(usually in the plural) furnishings and equipment (especially for a ship or hotel)
	making or becoming suitable; adjusting to circumstances

add-on (adjective)	fitting (adjective)
	being precisely fitting and right
	in harmony with the spirit of particular persons or occasion