In digital audio systems, particularly CD and R-DAT, additional data interleaved with the audio information which carries synchronization and user information, such as tags and comments which is independent from the audio data. The three main types of DAT subcodes are Start IDs which mark the beginning of each song, Program Numbers which are the ordinal numbers assigned to each Start ID, and Skip IDs which causes a player to not play the selection whose number is the current Start ID and go to the next Start ID. See also PQ Codes.