postscript vs appendix

postscript and appendix both are nouns.

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

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

postscript: 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 postscript include continuation, sequel.

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