additive vs add-on

additive and add-on both are nouns.

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

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

Other hypernyms of additive include addition, improver.

additive (noun)	add-on (noun)
something added to enhance food or gasoline or paint or medicine	a component that is added to something to improve it
	a supplementary component that improves capability
	a software module that extends or enhances the capabilities of an existing application

additive (adjective)	add-on (adjective)
characterized or produced by addition
designating or involving an equation whose terms are of the first degree