significance vs subject matter

significance and subject matter both are nouns.

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

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

Other hypernyms of significance include content, message, substance.

significance (noun)	subject matter (noun)
the quality of being significant	content communicating a message; what something is about
a meaning that is not expressly stated but can be inferred
the message that is intended or expressed or signified