If it is a right angled safety relief valve/pressure relief valve, then the outlet static pressure is the pressure acting on the valve outlet at full flow. It takes account of the actual relieving capacity, the molecular weight, temperature and outlet (not nozzle) area etc. This is all required to calculate the reaction force. You can find this in API-RP-520 or any reputable PRV manufacturers Engineering Manual.
Per ISO, only the term Safety Valve is used regardless of application or design.
