I used %_filename% in "Text file - Tag" format string to rename files from a list, but it did nothing. What am I doing wrong?
as the filename is the identifier to align a record with a filename, it cannot be changed with an import. You would have to do that with an action (afterwards).
Mp3tag supports writable "true" tag-fields like TITLE or ALBUM ...
and offers read only "pseudo" tag-fields like _PATH or _FILENAME.
Using the converter "Textfile - Tag" only writable "true" tag-fields can be filled with values from the import text file.
While importing values from the import text file into the media file it is not possible, to change the values from read only "pseudo" tag-fields.
The import text file does not need to have necessarily a binding value between the current text line and the target file. If there is none, then Mp3tag reads successively one text line for each file, and puts the given values as defined in the "import formatstring" into the files, in the order as the files are listed in Mp3tag list view.
If each line of the import text file contains a "binding relation" like filepath or filename, then Mp3tag can use this information, as defined in the "import formatstring", to address the target file in the Mp3tag list view, directly by using the given "binding value".
When the import has been done, it is possible to change the media file's filename to a new value, which has been imported beforehand into a temporary "true" tag-field.
For example ...
Convert "Textfile - Tag"
... using import file:
Convert "Tag - Filename"
... or ...
Convert "Tag - Tag" or Action "Format value"
Remove tag field FN_NEW.