About Mp3tag Tools feature and examples of different tool configurations ... read there ...
/t/966/1
Using a well known "environment variable" has advantages in contrast to hard-coded paths and filenames.
Good question.
Calling an application, which is made for invoking one file only, to be opened for multiple instances, ...
all together at once, for example for ten or hundred or thousands of files, ...
can make the computer system unusable for user intervention.
DD.20150214.1958.CET