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MPEG user data is data outside the MPEG specification that is muxed into an MPEG stream. User data can be inserted on three levels: the sequence level, the group of pictures (GOP) level and the picture data level. Applications that process MPEG data do not need to be able to parse this data but should be able to preserve it.
DVB encoders use the picture user data to indicate the aspect ratio of the stream, while ATSC encoders use the picture user data to indicate portions of the video stream that should be hidden. ATSC and other organizations have defined formats for embedding a Closed Captioning stream in user data, the ATSC standard is called EIA-708.
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