I know I am going to reveal myself to be ignorant about using MP3TAG, although I have been using it for years, but I cannot find an answer to this question, and would appreciate any help.
I spend a lot of time cleaning up other peoples' tagged files: the information may be there, but not necessarily in the right tag or format (for me).
As an example, for audio books, I like to put the "Track#-Album" into the Title tag (eg, "01-To Kill A Mockingbird"), and the Track# tag exists, and the Album tag exists. I do this so the Titles sort on my MP3 player, and I can see which Track it is from the Title.
The only way I have figured out how to do it, is to do a "Convert -- Tag - Filename", put "%track%-%album%" as the mask, and temporarily rename the files.
Then, I immediately follow that with a "Convert -- Filename - Tag", and use the "%title%" as the only mask.
The bottom line: I am using the Filename as a temporary location, because I cannot figure out how to go directly from one or more tag values, to another tag. Is it possible, and without re-typing everything each time (that is, be able to pick the masks like I currently do)? Obviously I have to screw up the Filename during this process, but usually I am cleaning up everything anyway, and renaming the files will just be the last thing I do.
Please excuse my ignorance, and many thanks for your help,