good afternoon vs salutation

good afternoon and salutation both are nouns.

Word	Noun	Adjective	Verb	Adverb
good afternoon	Yes	No	No	No
salutation	Yes	No	No	No

As nouns, salutation is a hypernym of good afternoon; that is, salutation is a word with a broader meaning than good afternoon:

good afternoon: a conventional expression of greeting or farewell, used to wish someone a good afternoon
salutation: (usually plural) an acknowledgment or expression of good will (especially on meeting)

Other hypernyms of good afternoon include farewell, word of farewell, greeting.

good afternoon (noun)	salutation (noun)
a conventional expression of greeting or farewell, used to wish someone a good afternoon	(usually plural) an acknowledgment or expression of good will (especially on meeting)
	an act of honor or courteous recognition
	word of greeting used to begin a letter