That’s an excellent question, and one I don’t know the answer for
If I had to take a semi-educated guess, I’d guessing that Maildir/.Junk/ would be what you want.
If you chose "Write to other file", you could go with the last one there, just specifying the full path.
If you want to know for sure, you could try one of two things:
File a request in the request tracker, ask Jamie exactly how it’s supposed to work, and mention that perhaps the documentation for those options could use some further explanation (or actually, even just examples would help).
Pick one and try it out It’ll write the file somewhere, so if it’s the wrong place, you can always just mv it into the proper location.
-Eric