sequel vs addendum

sequel and addendum both are nouns.

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

As nouns, addendum is a hypernym of sequel; that is, addendum is a word with a broader meaning than sequel:

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

Other hypernyms of sequel include postscript, supplement.

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