Most of the real work in the outer layer of the publishing framework is performed by its callback routines. The basic idea is that compose_for_type is passed the names of four outer layer subroutines, which it calls at predetermined points during execution. Each callback routine is passed the parameter array, and callbacks are expected to modify the name and value parameters in the array to control downstream processing.

Names of the callback routines can vary substantially between comp_type modules. The actual callback function names are passed as strings to compose_for_type. A callback name may be passed as an empty string, in which case compose_for_type skips the callback and proceeds as if it returned successfully.