You can also use all the global predefined variables and your own user-defined variables in the captions in the Table of Contents, keywords, image captions, link captions, macros and scripts, HTML code objects, HTML templates, PDF print manual templates and text entry fields in your project's Configuration section.
There is one difference, however: In all these locations you cannot use the Write > Insert Object > tool, you must type the variables in manually.
If you use HTML variables anywhere except in HTML templates and HTML code objects only the plain-text portion of the variable value will be inserted. If the variable does not contain a plain-text portion nothing will be inserted.
Variables and Conditional Output (Reference)