how-do-you-do vs salutation

how-do-you-do and salutation both are nouns.

Word	Noun	Adjective	Verb	Adverb
how-do-you-do	Yes	No	No	No
salutation	Yes	No	No	No

As nouns, salutation is a hypernym of how-do-you-do; that is, salutation is a word with a broader meaning than how-do-you-do:

how-do-you-do: an expression of greeting
salutation: (usually plural) an acknowledgment or expression of good will (especially on meeting)

Other hypernyms of how-do-you-do include greeting.

how-do-you-do (noun)	salutation (noun)
an expression of greeting	(usually plural) an acknowledgment or expression of good will (especially on meeting)
an awkward situation	an act of honor or courteous recognition
	word of greeting used to begin a letter