pub unsafe static AVTrackAssociationTypeAudioFallback: &'static AVTrackAssociationTypeAVAssetTrack only.Expand description
Indicates an association between an audio track with another audio track that contains the same content but is typically encoded in a different format that’s more widely supported, used to nominate a track that should be used in place of an unsupported track.
Associations of type AVTrackAssociationTypeAudioFallback are supported only between audio tracks. This association is not symmetric; when used with -[AVAssetWriterInput addTrackAssociationWithTrackOfInput:type:], the receiver should be an instance of AVAssetWriterInput with a corresponding track that has content that’s less widely supported, and the input parameter should be an instance of AVAssetWriterInput with a corresponding track that has content that’s more widely supported.
Example: Using AVTrackAssociationTypeAudioFallback, a stereo audio track with media subtype kAudioFormatMPEG4AAC could be nominated as the “fallback” for an audio track encoding the same source material but with media subtype kAudioFormatAC3 and a 5.1 channel layout. This would ensure that all clients are capable of playing back some form of the audio.
See also Apple’s documentation