A subject, or subject-matter, is a collection of tropes which frequently re-occur in tandem.
This term is intended to be differentiated from Genre, which the subject is included in.
Basically, if a bunch of tropes tend to show up together frequently: they’re a subject-matter. The subject is then, in other words, what the story is literally about. While the Theme is a more abstract idea.
For now, the only subject that I’ve positively identified is Mecha which I previously confused for a genre/sub-genre.