Inventing and assigning new "Genre" values possible?

[pstein]

Assume I invent a new "Genre" e.g. "foobar".
As far as I can see this Genre is accepted by MP3tag and entered into ID3v2.3 header.

But is this persistent?

Can this new Genre read by other audio players/tools or do they show "unknown genre" or similar?

Or is what is shown in Genre column just a human readable label for numeric encoded limited number of pre-defined genres?

[ohrenkino]

See here how to add non-standard genres:

Add custom user-defined Genres

Add and Edit user-defined Genres via Custom List Values Every field on the Tag Panel can have custom values added to its drop-down list. These can be customized via Options → Tag Panel and the Genre field is no exception. You can use Options → Tag Panel → Genre → Custom list values → Manage... to edit the list of custom Genres and also hide the standard genres, if you prefer your own. The following screenshot exemplifies the process: [User-defined Genres] Import and Export of Custom Lis…

Programs that follow the ID3V2 standard should show the genres as found in the files.
Programs that can only deal with ID3V1 tags are limited to the standard genres.

[LyricsLover]

@pstein

Or is what is shown in Genre column just a human readable label for numeric encoded limited number of pre-defined genres?

This was true for Genres entered in ID3v1 tag format. A fixed list with 80 entries (from 0 = Blues to 79 = Hard Rock was defined, later some more Genre has been added from 80 = Folk to 125 = Dance Hall).

Currently, using ID3v2 tag format, you can use as Genre whatever you like. It is just a free-form text.