Which tag fields are supported by Mp3tag?
You can edit all tag fields that are listed at Tag Field Mappings and add your own fields, which are then treated as user-defined fields.
To edit tags that are not listed in Mp3tag's Tag Panel (the input panel on the left), you can use View > 
Extended Tags... or Alt+T. If you want to have a specific field available on the Tag Panel, you can add it via Options > Tag Panel.
You can also add additional file list columns for extended tag fields
via View > Customize Columns... and edit the tag fields directly in the file list. Customizing the file list columns gives a brief introduction to the available fields.
Again, you can find a list of all standard tag-field names and their mappings to other tag formats and applications at Tag Field Mappings.