PS vs annotation

PS and annotation both are nouns.

Word	Noun	Adjective	Verb	Adverb
PS	Yes	No	No	No
annotation	Yes	No	No	No

As nouns, annotation is a hypernym of PS; that is, annotation is a word with a broader meaning than PS:

Other hypernyms of PS include notation, note.

PS (noun)	annotation (noun)
a note appended to a letter after the signature	the act of adding notes
	a comment or instruction (usually added)