Static AVTrackAssociationTypeMetadataReferent 

pub unsafe static AVTrackAssociationTypeMetadataReferent: &'static AVTrackAssociationType

Available on crate feature AVAssetTrack only.

Indicates an association between a metadata track and the track that’s described or annotated via the contents of the metadata track.

This track association is optional for AVAssetTracks with the mediaType AVMediaTypeMetadata. When a metadata track lacks this track association, its contents are assumed to describe or annotate the asset as a whole. This association is not symmetric; when used with -[AVAssetWriterInput addTrackAssociationWithTrackOfInput:type:], the receiver should be an instance of AVAssetWriterInput with mediaType AVMediaTypeMetadata while the input parameter should be an instance of AVAssetWriterInput that’s used to create the track to which the contents of the receiver’s corresponding metadata track refer.

See also Apple’s documentation