Some common types of propane cylinder fittings include OPD, ACME and POL valves. The POL valve is an older style that is falling out of use.
POL valves may still be seen on old pieces of propane equipment. This type of valve freely releases propane gas if the valve is turned, even when the fitting is not connected to any equipment. However, newer fittings are designed to prevent this from happening.
The ACME valve is a bulkier fitting that is still compatible with older equipment that uses the POL valve; it must be securely fitted to equipment before propane will flow from the valve. An OPD valve is similar, with the addition of overflow protection when filling the tank to prevent potential damage to the tank from placing too much propane into it.