The safety function is a feature of the actuators, which is mandatory by law or customer requirement, to close or open the valve in case of a power failure. It often happens that the power is out and the valve is therefore not under control anymore. This could lead to catastrophic consequences for the application/industry and most importantly for the safety of everyone in the vicinity of the valve as it can have more than 130°C of pressurized water in the system.
The safety function enables you to close or open the valve automatically (via the spring inside the actuator) each and every time the power goes out. SD means spring down, which will move the actuator to lowest end position, SU means spring up which will move the actuator to highest end position. You can find both versions in the Danfoss portfolio.