My wife does the tagging so I don't know the details. But it's ID3 tags and she sets a custom "mood" tag. So it might look something like this "genre: classical, instrumental" but another tag "mood: chill; quiet". Many players don't recognize this but it originally came from her using musicbee on windows. We do set multiple values in genre but she likes having the separate mood one as well. And we use rating which is another some player ignore. All of these she uses to build smart playlists.