add-on vs attachment

add-on and attachment both are nouns.

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

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

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

add-on (noun)	attachment (noun)
a component that is added to something to improve it	the act of fastening things together
a supplementary component that improves capability	the act of attaching or affixing something
a software module that extends or enhances the capabilities of an existing application	faithful support for a cause or political party or religion
	a supplementary part or accessory
	a connection that fastens things together
	a writ authorizing the seizure of property that may be needed for the payment of a judgment in a judicial proceeding
	a feeling of affection for a person or an institution