I had to look this one up in the code. It apparently will contain the field name when exporting from Single Field view mode. When exporting from Normal view mode, it's empty (as you've noticed).
The answer to where you'll find the list of tokens that are included in the export is in the user manual included with the software (accessed via the Help tab, or by pressing F12, or by using the shortcut installed on your Start Menu). But, as implied by my first sentence, this one is missing from the user manual. I'll make sure I add it.