postscript vs sequel

postscript and sequel both are nouns.

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

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

postscript: 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 postscript include appendix, continuation.

postscript (noun)	sequel (noun)
a note appended to a letter after the signature	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