What is stored there?
Nothing. It is there for compatibility with early CD players.
brief summary of “Red Book Compact Disc Digital Audio”… The CD format was designed in the 1970s before the widespread availability of cheep microprocessors. So you don’t need a computer of any kind of play a CD. So, audio CDs are structured like a simplex radio broadcast… or like a tape. There’s no concept of any organizational structure, like sectors, or an absolute position. There’s a lead-in and lead-out area (two seconds of alternating bits), at the beginning an end of the disk, so the player knows about where to start, stop, and when it has seeked out too far. And everything in between is just a bunch of ones and zeros you pump into your DAC. (With a bit of 14-bit to 8-bit and Reed-Solomon decoding, same as they were already using for satellite broadcasts in the 1970s).
Early CD players were not really designed to do random seeking when the user presses a button… According to the Red Book spec, the player only need to seek to about 2 seconds within wherever it is actually trying to seek to. That’s why a Red Book compliant CD must have 2 seconds of “pre gap” between each track. Early players would commonly ‘miss’ when you tell them to reposition their laser. The pre-gap has an extremely obvious bit pattern in the P subcode, which is how the player knows that it’s in the pre-gap, and not in the regular audio portion. The player mutes its audio while trying to locate the next track, skipping ahead a bunch, and the then checking if it’s in a pre-gap or not…
Source: CD tracks that have pre-gap audio - General chatter - MetaBrainz Community Discourse
Note: I am not able to verify this as:
The standard is not freely available and must be licensed. It is available from Philips and the IEC. As of 2013, Philips outsources licensing of the standard to Adminius, which charges US$100 for the Red Book, plus US$50 each for the Subcode Channels R-W and CD Text Mode annexes.
And I don't posses a copy of the standard.
Source: Compact Disc Digital Audio - Wikipedia