Found some hints here TiddlyWiki App: A Smooth User Experience (2025) for Multiple Platforms - #45 by oeyoews
I remember that the subwiki concept first appeared in TidGi-Desktop. It was designed to add a private folder for users. Even if the HTML is exported, the content in the private folder will not appear in it.
The usage is to add a private tag to a tiddler(in tiddlywiki-app)