garbage vs subject matter

garbage and subject matter both are nouns.

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

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

Other hypernyms of garbage include content, message, substance.

garbage (noun)	subject matter (noun)
a receptacle where waste can be discarded	content communicating a message; what something is about
a worthless message
food that is discarded (as from a kitchen)