As well as the main data channel on a CD, there are 8 additional subcode channels labelled P, Q, R, S, T, U, V and W interleaved with the main channel.

The P channel indicates the start and end of each track and is intended for use by simple audio players without complete Q decoding.

The Q channel supports all the standard goodies one expects from CDs -- minutes, seconds and frames, TOC details (in the lead-in), track type, catalogue number, etc.

Channels R through W are for graphics (CD-G) and/or text (CD Text) most commonly found on karaoke CDs.

Syncing subcode data back up with the main channel is one of the more common problems encountered when attempting to copy a CD -- see Why you can't bit copy a CD.