The Outlet Gauge screws into the Shutoff Valve Assembly. It is useful for diagnosing problems with the High Lifter and the outlet line. Use the 600 psi gauge for lifts over 500 feet. This is a very nice gauge that is glycerin-filled to resist corrosion and pressure shocks. It has a 1/4" pipe thread. It has a 2" face diameter and the mounting threads are on the bottom of the gauge.