addendum vs sequel

addendum and sequel both are nouns.

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

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

addendum: textual matter that is added onto a publication; usually at the end
sequel: a part added to a book or play that continues and extends it

Other hyponyms of addendum include appendix, continuation.

addendum (noun)	sequel (noun)
textual matter that is added onto a publication; usually at the end	a part added to a book or play that continues and extends it
	something that follows something else