General Information on user-defined Tools
User-defined Tools are external applications that can be added via Options > Tools to work directly from Mp3tag's main window via the context-menu of the file view (find more about this feature in the corresponding help topic.
You can also use the keyboard shortcuts Ctrl+1, Ctrl+2 to Ctrl+0 to trigger the first, second to tenth user-defined Tool on the selected files.
Parameters like %_path% should be in quotation marks, so paths and filenames with spaces are passed properly. An example of this would be Parameter1 Parameter2 "%_path%"