Editing DISCSUBTITLE tag

[Tassie Devil]

Sorry if this has come up before.
I have just loaded all CD's onto my PC and used several different programs to "tidy" them ... some have done severe damage to the file content and/or the sorting was bizarre.
Good I made backup folders so I could start again.
Fortunately MINIM on my QNAP NAS had given me a Log file where I could manually (very tedious) tidy up using Mp3tag. I still have some "errors" which indicate a problem with DISCSUBTITLE. I cannot see this in Mp3tag. Any suggestions for a suitable editor ?