I think emueyes might mean that the content of the ID3 tags might not hold the same info as the Vorbis Comments (FLAC tags, if you will). They are independent. So if he (she?) strips off ID3 tags, he will have to retag everything to add back info that is "missing" as a result. I think.
Since it sounds like he might have a lot of improperly tagged files (i.e. ID3 tags on FLAC file), he wants to save a lot of effort. Having Kodi ignore the improper files and make a log of which files failed would be the easiest, partial, solution for him. He could then just fix the broken ones, instead of going through every directory with a tool like mp3tag to batch fix/tag things.
However, that's just my opinion based on what I've read in this thread.
@emueyes:
The Wikipedia entries on vorbis comments and ID3 tags do a good job of explaining the tagging formats. Or rulesets, as you phrase it. Further than that, a little googling on the various filetypes and their structures would be required. Obviously, not every file is the same. Kodi just uses a library to read this metadata (see the previously linked trac ticket).
In truth, mp3tag has the ability to export the tags, en mass for multiple directories at once. You can even tag files from a text file. So you could export, flush all tags, and re-import, in bulk. So nothing would be lost. The program even has the ability to allow you to create your own custom export format.
So in theory, one could make a template to export the tag metadata to a
music NFO file that Kodi understands. Trivial for someone with years of programming experience.
Within a few seconds of googling, I came across an interesting
thread in another forum that speaks of ID3 vs vorbis tags in FLAC. A couple of solutions are provided that strip out just the ID3 tag info, leaving the vorbis tag alone. The third party "tag" command line utility is quite interesting and would be helpful, in my opinion, if someone wanted to fix things in bulk.
Good luck fixing your files.