It would be so so useful if you could amend all fields direct, specifically SUBTITLE, TVSEASON, TVEPISODE.
Thoughts?
Where do you want to do that?
Have a look at File>Options>Tag-Panel and add the fields in question there to the tag panel.
Or use View>Columns... to add columns for these fields.
New thoughts none but practice ==> use Mp3tag.
DD.20150814.1939.CEST
Not sure I made myself clear.
Background: I am using Mp3tag to manage MP4 videos with MP4 tags.
I have already added/removed columns as best suits my needs in the main window. Now, for example I can click on column 1 (Title) directly and amend the contents (much like Microsoft Excel if you can imagine: copy and paste, enter goes to next video in list etc). Now, column 2 (Subtitle) I cannot edit directly. I need to: right click, Extended Tags, New Tag blah blah blah... MUCH more time consuming. Make sense?
I wasn't aware the left hand panel can be customized though, so this is new to me. Is it possible to customize ALL the fields? I have no use for Album/Artist/etc but I do not see a way to remove those 'default' ones.
Customize column ...
Name: Subtitle
Value: %SUBTITLE%
Field: %SUBTITLE%
Sort By:
Numeric:
... should work.
See also ...
https://docs.mp3tag.de/mapping
DD.20150817.1812.CEST
Ah! Thanks! That works perfectly... 
So, any word on the left hand column? Is it possible to customise this too?
See File>Options>Tag Panel.
Sure... but is it possible to remove 'hard coded' fields. For example, I will never need to use 'Artist' in regard to MP4s.
I think you have seen the functions in File>Options>Tag Panel.
You can add fields or remove again the added fields.
I think the default fields represent the ID3V1 standard.
There are no 'hard coded' tag-fields in the usage with Mp3tag.
If you do not want to store any value into the tag-field ARTIST, then do not touch this tag-field.
Mp3tag does not store an empty tag-field.
An empty tag-field does not exist.
You can check the current status of tag-fields within a media file by viewing the dialog "Extended Tags..." [Alt+T].
If you want to remove a dedicated tag-field, then use the fitting action to remove this tag-field, ...
or do it manually within the dialog "Extended Tags..." [Alt+T].
DD.20150827.1805.CEST
Regarding the Tag-Panel dialog window, ....
where you can use self-defined input dialogs, ...
additionally to the displayed basic set of standard input dialogs, ...
if you do not fill any value into a input dialog, ...
then Mp3tag does not create the related tag-field.
Mp3tag does not write any empty tag-field into the tag within the media file.
DD.20150827.1815.CEST
I understand all this, I'm talking about the actual GUI of Mp3tag. I want to customize the fields in the left hand pane, but some appear to be forced/hardcoded/default. Whatever terminology you want to use.
Make sense?
No.
OK, thanks.
Maybe in the future...