Hi 
I am trying to improve support for a «disc subtitle» tag in various software including taggers such as Kid3 (I have an open PR for ffmpeg to add a mapping for such a tag).
Looking at the docs (and the source code), Kid3 does not support such a tag. It does however, have a «Subtitle» tag. Interestingly, it gets mapped to the TSST frame in ID3v2.4.
The official ID3v2.4 docs however, describe the TSST frame as «[…] intended for the subtitle of the part of a set this track belongs to.»
This leads me to believe that the TSST frame is not intended as a general «subtitle» but as a «set subtitle». Picard’s and taglib’s mapping come to the same conclusion.
Why is TSST mapped to Subtitle in Kid3? Would you be open to change the mapping? 