What is a Subchannel
Every Block on a CD is also accompanied by 8 subchannels (P, Q, ..., W). Every Subchannel can contain 12 bytes. The P and Q subchannels are used for time encoding and are very system critical. E.g. Copy protections tamper with these channels.
The other subchannels can contain data. E.g. CD-Text data or Karaoke data.