petition vs subject matter

petition and subject matter both are nouns.

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

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

petition: a formal message requesting something that is submitted to an authority
subject matter: content communicating a message; what something is about

Other hypernyms of petition include content, message, substance.

petition (noun)	subject matter (noun)
a formal message requesting something that is submitted to an authority	content communicating a message; what something is about
reverent petition to a deity

petition (verb)	subject matter (verb)
write a petition for something to somebody; request formally and in writing