The simple $ tag is used to indicate headings, sub headings, section headings etc.
How it works
Add the appropriate $ tag before the text. A single $ means a level one heading, $$ means level two, $$$ means level three etc. The range goes to level six i.e. $$$$$$.
$$ Sub heading
$$$ Section heading
$$$$ Sub section header
$$$$$ Sub-sub section header
$$$$$$ No idea what you’d use this level for
The book reader software will pick up the $ tags and create the right level of heading.
Mix ’em up
They can be used in any order and mixed up. For example, many print books declare the chapter sequence in smallish text (e.g. Chapter IV) and below it render the actual chapter name in bigger text. This is easy to accomplish in a UBook.
$ The Road to Nowhere
This would look something like this.