The quality of the permeate is being measured and as long as a set quality value (can be programmed in the controller) is not reached, the valve stays closed and the permeate goes back to the inlet. When the required value/quality is reached, the valve opens and the permeate tank is filled.
This is a relevant option when:
Reverse osmosis starts and stops frequently
A certain maximum limit value of the permeate in the storage tank needs to be guaranteed