The ASOV or autoshutoff valve senses back pressure when the float valve, float switch or in the case of a drinking water system, when the pressure tank is full and shuts the flow off to both the treated and the waste lines. They are around $10-$15 most places and make sure you get a check valve with it as it makes the ASOV work better.
I prefer a more robust system myself so in addition to the ASOV I use DC powered float switches and a solenoid instead of a simple float valve, I have seen too many cooler floats, toilet tank floats and swimming pool self levelling floats fail.
It is not advisable to hook a RO/DI directly to a sump or display in case the ASOV or float valve fails which will flood the system with unlimited fresh water. Instead fill a ATO storage container with only as much RO/DI as you would feel safe adding to your system without lowering the salinity so far it would kill everything then have your ATO system fill the sump or display from that.
I have my RO/DI filling a 23 gallon Rubbermaid can in my garage with float switch and solenoid valve control then my ATO pulls water from there about 35 feet into the house to my sump using a peristaltic pump and a digital level sensor. This has been in operation over 6 years now with zero issues.
