You can add and remove genres via the New and Remove buttons next the genres list. Editing user-defined genres is possible via the Edit button.
The genres added here are available via the drop-down list of the Genre field on the Tag Panel. Note that these genres are not saved to an ID3v1 tag due to the limitations of the tag format.
If enabled, the drop-down list of the Genre field on the Tag Panel only shows user-defined genres after a restart of Mp3tag.
If enabled, Mp3tag shows the genres from the tags in the list on the Tag Panel and if the files don’t have any, shows the list of genres to select from.