A Closed Captioning stream is a data stream hidden inside a video stream which represents the audio portion as text.
Sometimes MPEG2 elementary video streams have closed captioning data embedded in it, such as IEC-608. In such cases the MPEG2 Decoder generates two output streams: Video and Closed Captioning.
In addition, AVI and OGG container formats can have closed captioning data as a separate stream. For instance, an MPEG2 file with closed captioning is converted to an AVI format that will have three streams: Video, Audio and Closed Captioning.