add-on vs fixings

add-on and fixings both are nouns.

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

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

add-on: a supplementary component that improves capability
fixings: the accessories that normally accompany (something or some activity)

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

add-on (noun)	fixings (noun)
a component that is added to something to improve it	the accessories that normally accompany (something or some activity)
a supplementary component that improves capability	food that is a component of a mixture in cooking
a software module that extends or enhances the capabilities of an existing application