message vs subject matter

message and subject matter both are nouns.

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

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

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

Other synonyms of message include content, substance.

message (noun)	subject matter (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