Returns related resources of the playlist. Must be a comma separated string consisting of one or many: songs, songs.audio_files, songs.artists, playlist_categories. For more information on include see the (JSON:API spec)[https://jsonapi.org/]..

If true, all audio files for each song will be returned in the audio_files relationship, and in the top-level included key. If false, only primary audio files will be returned in the audio_files relationship, and in the top-level included key. By default, this is false, because response size will be larger when including all audio files for each song.