The code model puts constraints on the location of symbols and the size of code and data. The selection of a code model is a trade off on speed and restrictions that needs to be selected on a per application basis to meet its requirements. A slightly more detailed explanation can be found in (for example) the System V Application Binary Interface (x86_64) 3.5.1.
This data structure is used by the Zig language code generation and therefore must be kept in sync with the compiler implementation.