Airsoft guns are classified according to their operating principle, which can be spring-loaded, electric (battery-powered Automatic Electric Guns, Airsoft Electric Guns or "AEGs"), or gas-powered (if these have a blowback feature they are known as "GBBs"). Some companies produce full replicas of counterpart grenade launchers which fire a projectile spray of pellets by use of a high-powered spring mechanism or a compressed gas propellant (i.e. green gas, propane, compressed air, or CO2). There is also a type of airsoft gun that is powered by a single (HPA) high-pressure air tank and a battery pack to power the internal FCU "Fire Control Unit" computer board. These guns are often known for their high and adjustable ROF or "rate of fire" and durability. These HPA replicas are most often referred to as "PolarStars", although this refers to a popular brand of HPA engines and accessories.