aside vs subject matter

aside and subject matter both are nouns.

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

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

Other hypernyms of aside include content, message, substance.

aside (noun)	subject matter (noun)
a line spoken by an actor to the audience but not intended for others on the stage	content communicating a message; what something is about
a message that departs from the main subject

aside (adverb)	subject matter (adverb)
on or to one side
out of the way (especially away from one's thoughts)
in reserve; not for immediate use
not taken into account or excluded from consideration
placed or kept separate and distinct as for a purpose
in a different direction