For movies and TV shows, Kodi and some other software support IMDb and similar IDs stored either directly or as URLs pointing to the respective sites inside simple textual or XNL .nfo files that either have the same file name as the media file with a different extension or have a special name like tvshow.nfo.
I would like to store such IDs inside the media files themselves, i. e. in their MP4 tag. Which field(s) should I use?
By the way, Kodi v18 Leia (source code) does not seem to expect it in any particular field yet.