you can find full list of tags in mp3tag's help file. open mp3tag, hit F1 key, then click "Names and mapping of ID3v2/WMA tag fields". all other tags that are not lisited here are custom tags. you can name them however you like with spaces and underscores (but try to anvoid special and unicode chars, because i am not sure about their support though).
so called tags that starts with underscore like %_length% are not tags at all. they are file properties that you can use in tagging and exporting. some of them are read-only.
to see all tags within your mp3, mark the files that you need click view -> extended tags (ALT+T).
and finally %itunnorm% vs. %comment itunnorm%. if you want that tag to be read by other software (or hardware) name it this way, which your software supports this tag (if it uses %itunnorm% use it, if %comment itunnorm% use that accordingly). if not, it does not really matter how you will name your tag.