Subchannel (P–W)
Eight bit-streams (P–W) running alongside the audio on a CD, carrying timecode, track numbers and CD-Text.
Beyond the audio, every CD frame carries eight subchannel bits, named P through W. P flags the pause between tracks; Q is the workhorse, encoding track number, index and time as minutes-seconds-frames (MSF) — the numbers your player shows. Channels R–W sat idle for years before finding use in CD+G (karaoke graphics) and CD-Text (disc and track titles).
The subchannel is also how a player locates a track without reading the main data: it reads the Q channel's position while seeking.