This is the key that fits the valve of the keg. There are as many kinds of couplers as there are valves – called systems, each forming a perfect lock-key combination. The keg coupler connects the gas and beer lines to the keg through which the pressurized gas is pumped in and beer is pushed out into the beer line.
Branding is the reason behind this multiple keg coupler-valve systems. A keg system has come to be associated with a specific brewery and differentiated from others when kegs went for refilling.
Your beer can be served properly only with the correct type of coupler that fits the keg valve. Hence, it is best to contact your local distributor or brewery itself in case of any questions about the type needed.
In the USA, most breweries use the D system.