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.