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.