pub unsafe static AVTrackAssociationTypeSelectionFollower: &'static AVTrackAssociationTypeAVAssetTrack only.Expand description
Indicates an association between a pair of tracks that specifies that, when the first of the pair is selected, the second of the pair should be considered an appropriate default for selection also. Example: a subtitle track in the same language as an audio track may be associated with that audio track using AVTrackAssociationTypeSelectionFollower, to indicate that selection of the subtitle track, in the absence of a directive for subtitle selection from the user, can “follow” the selection of the audio track.
This association is not symmetric; when used with -[AVAssetWriterInput addTrackAssociationWithTrackOfInput:type:], the input parameter should be an instance of AVAssetWriterInput whose selection may depend on the selection of the receiver. In the example above, the receiver would be the instance of AVAssetWriterInput corresponding with the audio track and the input parameter would be the instance of AVAssetWriterInput corresponding with the subtitle track.
See also Apple’s documentation