When exporting, if two files exported to the same folder location would have the same name, Tinderbox creates a synthetic name to avoid the collision.
For example, if two notes in the same container are both named "1", the first will be exported as "1.html" and the second as "1_1.html". A third note will be exported as "1_2.html".
For this reason, Tinderbox will not return a value if testing $HTMLExportFileName, except if it has a value set explicitly. Even in the case of the latter, the stored name value will be modified according to the above rules should a naming collision occur.
For more com;lex export scenarios - and where it is assumed the user has taken care to avoid naming collisions, it is possible to get the exported filename (with its extension) via ^file()^.
This feature covers both HTML and Text export methods.