Mode 3/A is designated as the common military/civil mode for air traffic control use.
Make radar beacon code assignments to only Mode 3/A transponder-equipped aircraft.
Unless otherwise specified in a directive or a letter of agreement, make code assignments to departing, en route, and arrival aircraft in accordance with the procedures specified in this section for the radar beacon code environment in which you are providing ATC service. Give first preference to the use of discrete beacon codes.
SQUAWK THREE/ALFA (code), or
NOTE: A code environment is determined by an operating position’s/sector’s equipment capability to decode radar beacon targets using either the first and second or all four digits of a beacon code.