This is what the documentation says:
Value is required and denotes the content displayed in this column. It usually contains a placeholder , sometimes combined with scripting functions .
A simple example would be %ISRC%, the placeholder for the ISRC tag field. Another example would be $metasep(artist,\\), a format string that displays all existing ARTIST fields separated by Mp3tag multi-field separator \\
Field usually contains a placeholder that is used for editing the content of the field through the File List. Please note that if this field is empty, the column is write-protected. You can only use placeholders for tag fields (e.g. %ISRC%) as well as %_filename_ext% and %_filename% here.
Where do you have problems?
The field is called POPULARIMETER not RATING if that is what confused you.