subject matter vs message

subject matter and message both are nouns.

Word	Noun	Adjective	Verb	Adverb
subject matter	Yes	No	No	No
message	Yes	No	Yes	No

As nouns, subject matter and message are synonyms defined as:

subject matter and message: content communicating a message; what something is about

Other synonyms of subject matter include content, substance.

subject matter (noun)	message (noun)
content communicating a message; what something is about	content communicating a message; what something is about
	a communication (usually brief) that is written or spoken or signaled