This tool allows you to rename or create a new copy of a given MP3 file, based on the ID3V2 field values (artist, album, track number ...)
For the file renamer to work, you need at least a pattern to be defined and your files must have ID3V2 tags.
The patterns are quite similar to the ones in the tag editor, but here you cannot use relative file names (it's either "absolute" or "current directory") and you cannot have nested optional parts.
The "New file name" shows the full, absolute path for a file's new name. If you don't have permissions to create those files, the rename will fail.
My main use for this is to have consistent naming for files (my older files have a "-" or a "." between the track number and the track title, while the newer ones don't.)
Another use is to get reasonable names for files taken off some MP3 players.
Another possible use is to add ratings to the file name, so you can tell what the rating of a file is just by looking at the name, without having to search through your MP3 player's library. Then it may be easier to build custom lists with files that you like more. If it's easier or not depends on what MP3 player you use; I guess some already have this kind of functionality.
While the file renamer normally opens one folder at a time, it is possible to get its list populated with all files that are viewable in the main window. To do that, keep CTRL pressed when launching the file renamer.
Normalization 
in the main window to open this window.
