add-on vs additive

add-on and additive both are nouns.

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

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

Other hyponyms of add-on include afterthought, annex, annexe, extension, wing, attachment, elongation, extension.

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

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