There are two locations of identifying codes, one just above the top of the head which has a single code, and one at the upper chest area which has a set of a few codes (the number and configuration of these codes varies for different people), and both have the codes embedded in larger spaces. Both of these relate, to some degree, to the Master/slave area near the end of the Magick-universe, and can be completely erased by processing that area, the implants from the Between-lives area, and the origins of relevant energy-body structures in the Magick-universe as well as the structures themselves.
Another related location is the Piece-definition, which is a few inches below the base of the spine and also contains data. While this can be erased, if one’s Player is functional, it is typically better to allow one’s own Player to own oneself as Piece, which helps the Player function better than if it owned no Pieces.