fixings vs add-on

fixings and add-on both are nouns.

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

As nouns, add-on is a hypernym of fixings; that is, add-on is a word with a broader meaning than fixings:

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

Other hypernyms of fixings include accessory, appurtenance, supplement.

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