You can use both Help+Manual's predefined variables and your own user-defined variables in your topics and topic headers. These are all displayed automatically in the Insert Text Variable dialog so you don't need to refer to a list to use them.
If you use HTML variables in your topics and headers only the plain-text portion of the variable value will be inserted. If the variable does not contain a plain-text portion (text outside of any HTML tags) nothing will be inserted.
Variables and Conditional Output (Reference)