This article is from the CD-Recordable FAQ, by Andy McFadden (email@example.com) with numerous contributions by others.
There are eight subcode channels (P,Q,R,S,T,U,V,W). The exact method of
encoding is discussed in section (2-43), but it's really only important
to note the data is distributed uniformly across the entire CD, and each
channel can hold a total of about 4MB.
The P subcode channel identifies the start of a track, but is usually
ignored in favor of the Q channel.
The Q subcode channel includes useful information, which can be read and
written on many recorders. The user data area contains three types of
subcode-Q data: position information, media catalog number (MCN), and
ISRC code. Other forms are found in the lead-in, and are used to enable
multisession and describe the disc TOC (table of contents).
The position information is used by audio CD players to display the current
time, and has track/index information. This can be controlled when doing
The ISRC (International Standard Recording Code) is used by the recording
industry. It states the country of origin, owner, year of issue, and
serial number of tracks, and may be different for each track. It's
optional; many CDs don't use this. The media catalog number is similar,
but is constant per disc. Note these are different from the UPC codes.
for some details on P and Q.
The R-W subcode channels are used for text and graphics in certain
applications, such as CD+G (CD w/graphics, supported by SegaCD among
others). A new use has been devised by Philips, called ITTS. It enables
properly equipped players to display text and graphics on Red Book audio
discs. The most recent result of this technology is "CD-Text", which
provides a way to embed disc and track data on a standard audio CD.