What the TAG PANEL is for ?
The "Tag Panel" are the tag input fields on the left of Mp3tag's main window. You can use them to enter tag information like artist, album and title. Once you've changed anything here and want to save your changes to the files, press or use the [Ctrl+S] keyboard shortcut.
To save the same information to multiple files, just selected the file, enter the information (like the artist) at the Tag Panel and save your changes. Fields containing "< keep >" are not changed.
Hope that helps!