Alternate Source

This feature allows a provider to specify one of a set of variant playlists as an “override” of the main presentation. The client will only play the override media (audio or video), and suppress any media of the same type from the main presentation, if present. This allows a presentation to offer multiple versions of the media without requiring the provider to store duplicate media, or requiring the client download all variants when it only needs one. It also allows additional media to be offered subsequently without remastering the original content. Read more at Apple dev technote.