SO1/ SO2/ SO3
SO1 – Staff Officer 1 – Cdr/ Lt Col/ Wg Cdr level Appt
SO2 – Staff Officer 2 – Lt Cdr/ MAjor/ Sqn Ldr level Appt
SO3 – Staff Officer 3 – Lt/ Capt/ Flt Lt level Appt
Normally job titles rather than ranks but have become somewhat interchangeable in MOD/ Joint considerations. You can get Lt Colonels in SO2 jobs but its rare. Similarly, some jobs can be designated as being suitable for SO2/3 to give Appointing flexibility.