addendum vs appendix

addendum and appendix both are nouns.

Word	Noun	Adjective	Verb	Adverb
addendum	Yes	No	No	No
appendix	Yes	No	No	No

As nouns, appendix is a hyponym of addendum; that is, appendix is a word with a more specific, narrower meaning than addendum:

addendum: textual matter that is added onto a publication; usually at the end
appendix: supplementary material that is collected and appended at the back of a book

Other hyponyms of addendum include continuation, sequel.

addendum (noun)	appendix (noun)
textual matter that is added onto a publication; usually at the end	a vestigial process that extends from the lower end of the cecum and that resembles a small pouch
	supplementary material that is collected and appended at the back of a book