message pad vs pad of paper

message pad and pad of paper both are nouns.

Word	Noun	Adjective	Verb	Adverb
message pad	Yes	No	No	No
pad of paper	Yes	No	No	No

As nouns, pad of paper is a hypernym of message pad; that is, pad of paper is a word with a broader meaning than message pad:

Other hypernyms of message pad include pad, tablet.

message pad (noun)	pad of paper (noun)
a pad of paper on which messages can be written	a number of sheets of paper fastened together along one edge