This flow restrictor is installed in the black waste line of a RO/DI system. Fits 1/4" RO tubing.
There are different sized flow restrictors; they must match the membrane they're paired with. This one is designed for the 100gpd membrane.
If you are replacing a previous flow restrictor:
Make sure you remove the black tubing completely from the membrane housing and check to see if there is a flow restrictor inserted within that tubing. If present, remove it. Then reinstall black tubing. Cut the tubing 3" from the membrane housing fitting, and install the new inline flow restrictor, then reconnect the rest of the black tubing to the other end, making sure the arrow is pointing in the direction water is meant to flow.... toward the drain.