2.1 CLASS A: FREE BALLOONS 2.1.1 Class A, free balloons shall be divided into five sub-classes, each containing 15 categories according to size. The sub-classes are the following: 2.1.1.1 Sub-class AA: Free balloons, not equipped with an airborne heater, which obtain their buoyancy from a lighter-than-air gas, without pressurisation of the envelope. 2.1.1.2 Sub-class AX: Free balloons which obtain their buoyancy solely as a result of heating air. The envelope may contain no gases other than air and the normal products of combustion. 2.1.1.3 Sub-class AM: Free balloons which use both a lighter-than-air gas and an airborne heater, without pressurisation of any envelope. 2.1.1.4 Sub-class AS: Free balloons which use a lighter-than-air gas and are designed to allow sufficient pressurisation of the envelope to affect performance substantially. 2.1.1.5 Sub-class AT: Free balloons not falling into subclasses AA, AX, AM or AS