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Syntax for such information is specified in ISO/IEC 23002-3:2007 as a stream of data referred to as a supplemental information (SI) message stream. Provisions for extensibility have been included, so that additional types of data can be defined in future extensions of the current SI message stream syntax by ISO/IEC.
An SI message stream can contain several concatenated SI messages, hence conveying various types of information. The auxiliary video SI (AVSI) is the only currently-defined type of SI (other than reserved SI message types that are reserved for future specification by ISO/IEC and are to be ignored by decoders if present). An AVSI message characterizes the interpretation of an auxiliary video sequence that accompanies a primary video sequence. For instance, an AVSI can indicate that the auxiliary video represents depth map information, and can provide parameters for the proper interpretation of the auxiliary video as such depth information. The means for identifying the primary video stream and the auxiliary video stream to which these messages pertain is a system-level issue that is outside the scope of ISO/IEC 23002-3:2007.
Although the auxiliary video SI is the only type of SI that is currently specified in ISO/IEC 23002-3:2007, the SI message format has been defined in a generic fashion so that it can potentially be used for purposes other than aiding in the interpretation of auxiliary video sequences. Any kind of data could potentially be carried in the SI message format.