The inlet to the ro/di unit is tap water, it can be hoocked up a # of ways, from the faucet with an adapter, a pierce valve on a cold water copper line after the water softner if you have one, a unit consist of a sediment prefilter to get small particles out, then a carbon block filter to get out clorine/chlorimine and a few other things, then the water goes thru a reverse osmosis membrane, at that point it should almost pure, about 3 total dissovled solids, TDS for short, then it goes thru the DI, it is a resin that attaches the rest of the ions in the water too, after that the water should have 0 TDS, I for got what DI stands for. No impurities, chlorine, phosphates, ect, use it for top off/make up water from evaperation and to mix with your salt for water changes.
