What resource types are supported by MusicKIt addToLibrary function?

Question

My code for adding songs to user's music library looks like this:

let kit = MusicKit.getInstance();
kit.authorize()
    .then(() => {
        kit.addToLibrary('1234567', 'SONGS');
    });

Do you know what other types, besides SONGS, are supported? Apple's documentation of addToLibrary function is not very informative.