Modbus set digital output mask register

I wounder how works the Modbus output bit mask registers ?

Adress 1, Read/Write, Outputs
Adress 2, Write, SetOutputsBitsMask
Adress 3, Write, ClearOutputsBitsMask

In RTDE, there is a standard_digital_output_mask to decide whether or not to control the digital output.
I suppose it is a related function but I found no explicit documentation.