This adjustable pressure relief valve is used when 

 1. transferring beer between two kegs

2. Used when filtering between two kegs

3. to relieve pressure when naturally fermenting or carbonating beer in a keg

The valve is adjustable between 0 and 15 psi.

The pressure relief valve connects to the gas post  using a mfl threaded gas in disconnect.

to transfer between kegs: Connect the relief valve to the gas post of the receiving keg and gently adjust the valve to allow headspace to vent at the desired rate as the keg fills up. This can relieve oxidisation of your beer  by purging the recieving keg with CO2 first and venting as it fills.

 Also helps to reduce foaming and  loss of carbonation if it is already carbonated.

The valve can also be used if you want to naturally carbonate your beer in the keg. Simply adjust the valve to vent at the desired pressure based on how many volumes of CO2 you want in your beer.

Note: You need to wind/unwind the end of the valve to adjust the pressure. It sometimes comes threaded all the way on from the manufacturer. Wrap a towel around the end piece(about 1/4 in) and clamp down on it while twisting the wrench fitting on the bottom(closest end to the Gas In Quick Disconnect) to loosen it. Then finger tighten or loosen to desired pressure.