Hi Roy,
The /RSTOUT pin of the AD5764R is an output pin. I am not sure about which voltage it is being watched but it is most probably the AVDD that is being monitored. The part has a power-on reset circuitry that ensures the data registers are loaded with 0x0000 at power-up. The same circuitry also monitors the power supply voltage and makes sure that the power supply is high enough for the internal circuitries of the part to function properly.
You may use this pin to also control external system components, for example, a gate of a transistor or a pin of another IC.
Please let me know if you have any further questions.
Regards,
Mark