A Sub-topic is what we call a page (
Tiddler) that has been
tagged with the title of another tiddler; being tagged by 'A' denotes 'B' as standing
underneath Topic A in the hierarchy
That other tiddler (if you are
Editing it is the title you type in the "tags" box) is known as the
Topic, and is the 'headline' over any other tiddlers that it tags.
See the
Show references and info panel to reveal lists of topics and sub topics.