Static AVTrackAssociationTypeRenderMetadataSource 

pub unsafe static AVTrackAssociationTypeRenderMetadataSource: &'static AVTrackAssociationType

Available on crate feature AVAssetTrack only.

Indicates an association between a metadata track and another track where the metadata provides additional information for rendering of that track.

This track 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 for the target track that would be rendered (for example, a video track).

See also Apple’s documentation