Closed captions are part of the data packets of the MPEG-2 video streams inside of the video transmission stream.
Closed captioning is supported to the extent provided by the AV Foundation framework.
Closed captioning allows people with hearing disabilities to have access to video programming by displaying the audio portion of the video as text on the screen. Closed captioning differs from subtitles in that subtitles are typically not in the same language as the audio and subtitles do not typically include information about background sounds such as the sound of a door slamming or music.