topic vs subject matter

topic and subject matter both are nouns.

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

As nouns, subject matter is a hypernym of topic; that is, subject matter is a word with a broader meaning than topic:

Other hypernyms of topic include content, message, substance.

topic (noun)	subject matter (noun)
some situation or event that is thought about	content communicating a message; what something is about
the subject matter of a conversation or discussion