Hi,
Measure the 33V at full load.
Then take 90% of this voltage (maybe less on unstable mains grid) and take this is early_power_fail threshold.
This signal often is used to tell an MCU (via IRQ) to expect a power loss soon and safe all necessary data before power off.
I don´t think this is usefull. Why swithing power OFF erlier than necessary.
But you may use it for power sequencing or soft start reasons.
If your application doesn´t state this in the datasheet. --> don´t care about it.
It simply prevents the 22V_supply_voltage to drop below (5V_supply_voltage - 0.7V)
Klaus