Quick answer- I’ll find out the exact details for you shortly- we (HK CAD) are allocated certain blocks of codes to try to avoid duplication in the region as far as practicable. Obviously the aircraft within our surveillance area need to all have discrete codes to allow automatic correlation by the radar data processor. If not, we end up with an ambiguity and ‘Dupe’ code indication.
For departing aircraft, our radar system automatically allocates the next available code from the appropriate bin at clearance issue stage.