public MusicRecording getTrack() {
    return track;
  }
The index of track inside the playlist. Best efforts to make sure the correct order of playlist
items. Optional. Any int is valid - these numbers are meant purely to be relative to other
MusicPlaylistItem {@code order} values
portability-types-common/src/main/java/org/datatransferproject/types/common/models/music/MusicPlaylistItem.java