All TRANSMIT and RECEIVE exits must follow standard linkage conventions. They must save registers on entry, and restore all registers, except register 15, on return. The exit must be reentrant, refreshable, reusable, and reside in an APF-authorized library. APF-authorization enables the exit to use restricted authorization checking functions (for example, RACHECK). It is inadvisable for the exit to have APF authorization unless it is designed also to be called as the entry point for a job step program.