What is the difference between the rated and withstand Voltage?
The withstand voltage is explained in the specification sheet for your part:



The withstand voltage is the maximum voltage where the manufacturer guarantees less than 1 mA leakage current and no damage to the part if it is applied for up to 5 s.

The "rated voltage" is 40% of the withstand voltage. Probably they intend the rated voltage to be the maximum nominal operating voltage for the part.