Adding tags to "Tags" in the Description section

I can use it to change the Title tag, I can change the Subtitle tag, I can change the rating tag and the comments tag but I can't change the tags tag.

MP3TAG is clearly faster for changing tags than windows explorer, so I would like to be able to change these tags within MP3TAG if it is possible but I can't see it in the list.

Unlike the WE, MP3tag shows only fields that have contents, the others are actually not there.
So it would be best if you filled the field that you want to see in MP3tag in WE first, then load the file in MP3tag and press Alt-T to open the extended tags dialogue.
There you should see the list of currently existing fields and their contents.

That didn't work :<
I don't think it's in there which is ironic for an app with Tag in the name that is suppose to be used to fill in tags and the one kind of tag it seems to not be able to change is called TAGS

see also

And just to get it right: that what MP3tag deals with is the whole metadata which all in all is the tag.
This metadata is then divided. In most cases we call it "fields" (but could also be chunks, atoms etc).
You can also see TAG in Mp3tag (%_tag%) but this refers to the tag version like Id3V2.3, FLAC, APE.