I find it a little confusing that this one field has two names that suggest quite different intended uses. If you use this field, what do you use it for? I looked for documentation on the standard(s) and came up dry. id3.org seems to be broken at the moment.
I'm thinking about using this instead of COMMENTS for additional metadata to set up smart playlists, like the language the lyrics are in, the original physical format (shellac, vinyl, cd, etc), stuff like that. But, I might just put this info in COMMENTS.
The ID3v2 spec denotes this field as Subtitle/Description refinement, so I think you're good to go and the intended uses are not so different.
You can also use user-defined fields for the language the lyrics are in (or LANGUAGE), the original physical format (or MEDIATYPE) and use Convert → Tag - Tag to compose the contents of SUBTITLE or COMMENT.
That site has been broken for many months.
I have attached a zip file that contains an HTM file taken from that site that explains the standard. id3v2.3.0 - Specifications.zip (332.6 KB)
Often the player software will be the limiting factor here. Most do not have support for fields outside of the “normal” ones commonly used, like Artist, Album, Title, etc. Some do of course, but typically only those running on computers. Mobile device apps not very likely.
Yes, that is an issue. Apple Music does not show the LANGUAGE field, for example, nor any user-defined fields. But I started my audio collection in it and have not found a better player yet for filtering and smart playlists, even though Apple Music's UI gets worse with every release (for example, they changed song search from filtering the file list to one that just shows giant icons for the songs, with no organization at all!).