The “Water in” is the main inlet from the hose adapter.
The “Pure water out” goes to your fish tank, or a separate storage tank for the RO/DI water.
The “RO only” would go to a drinking water storage tank, then to a faucet.
The “Drain” is the wastewater & goes to the drain saddle that connects to the sink drainpipe.
There should be little blue shipping caps in the connections! To remove them you will need to push in on the little rings while pulling the caps out. To install the line you just need to push it into place. Hope This Helps
