Wiktionary
subtopic
n. A subject that forms part of a topic.
Usage examples of "subtopic".
Seventy-seven entries found under Tactical subtopic, one-hundred fifty-one entries found under Federation/Klingon Liaison sub-topic.
He'd rushed ahead on a new subtopic before he'd discussed, with Herring, the chapter he'd handed in the previous week.
Now you could say—if you were an editor—that the one thing the American reading public doesn't need foisted on them is another story about Going Mad Stylishly in America, subtopic A, Nobody Talks to Each Other Anymore.