This little thing is a hydrostatic valve. It goes in the bottom of your pool to stop the pool from floating (or buckling) if the water level on the outside of the pool is higher than the level on the inside.
It doesn’t have a rated flow rate or a rated differential pressure.
But if it doesn’t work your pool might float out of the ground.
I used to think rainy days would fill a pool as quickly as the ground around a pool hence there wouldn’t be a need to worry.
But if you have bad site drainage and water runs towards your pool there is a very real chance your empty pool could float.
Whether you could claim for damage to your pool from ‘flood water’ depends on your insurer – but we all have access to aerial photos and we can tell if your pool has been empty for a long time.
Bottom line: take the hydrostatic valve out of the bottom of your pool when it is empty. Only put it back in when you intend to fill your pool up.