how to understand the permissible voltage and current of film capacitor?
Excess voltage will cause the capacitor to fail due to dielectric breakdown.

Excess current will cause the capacitor to fail due to overheating (because of the equivalent series resistance of the capacitor). Since an AC voltage implies an AC current as the capacitor charges and discharges, an AC voltage can lead to overheating failure at a lower voltage than the DC voltage that would cause dielectric breakdown.

So you have two different failure mechanisms, and you need to consider different limitations to avoid these two types of failure.