Absolute Time in Pregroove (ATIP) is a method of storing information on an optical medium, used on CD-R and CD-RW.
ATIP is used as a method of putting data on an optical medium, specifically: These features are rather important to the function as it lets the drive know if it is writable.
The read/write laser in the drive will follow this pre-groove track as it is writing, in order to maintain the spacing between the written tracks with a high degree of accuracy.
This spiral has a wobble frequency introduced into it, which the drive can read while it is writing.
This wobble is further modulated with a timecode reference signal allowing the drive to approximately locate specific blocks of data on the disc.