1. A machine readable non-transitory storage medium storing instructions and data comprising: a time based metadata track associated with a time based media track containing at least one of audio data or video data, the time based metadata track comprising a set of identifiers that specify the one or more types of metadata in the time based metadata track; a non-time based description of types of metadata in the time based metadata track, the non-time based description comprising the set of identifiers that specify the one or more types of metadata in the time based metadata track, each of the identifiers in the set of identifiers uniquely specifying a type of metadata relative to all other types of metadata in the metadata track.