The open-drain output lets the designer wire-OR the reset signal with other supervisors or pull it up to a different logic voltage. That 125°C ceiling is the junction, not the case — the SOT-23-6 package's thermal resistance to ambient (RθJA) will be the limiting factor in a high-power-density layout.
Adjustable threshold and timeout — one resistor sets the trip
Both the voltage threshold and the reset timeout are adjustable/selectable via external resistors. The threshold is set with a resistor divider from the monitored rail to the sense pin; the timeout is programmed by a capacitor from the CT pin to ground. This flexibility means the same base part number can serve a 3.3 V rail in one design and a 1.8 V rail in another, with the reset hold time tuned to the downstream processor's startup sequence.