The density of both gases and liquids depends on the pressure and temperature of the fluid. Thus volumes measured in cold conditions or in pressurised comditions will be lower than the same mass of fluid at warmer or depressurised conditions. This fact gives rise to the necessity of choosing a benchmark pressure and temperature in which all 'net' volumes will be expressed.
The density of a gas at a specific pressure can be estimated by using the ideal gas law. Doubling absolute pressure doubles the density of a gas, and doubling absolute temperature halves the density. The number of molecules in a given gas volume depends on the pressure and temperature. This is why the pressure and temperature must be stated in order for a volume measurement to mean anything.
Density correction is also performed on liquids under measurement. For instance, the sticker on gasoline pumps that states that the volume is corrected to 15 °C (59 °F, or 60 °F in the U.S.) means that the measured volume has been compensated for thermal expansion. One would otherwise get a larger mass of gasoline in a tank filled in cold weather (which is unfair to the business) and less when it is warm (which is unfair to the consumer).
Legal regulations usually require that sales to end customers be compensated to the legally defined base conditions. Where regulations do not require the use of a specific set of base conditions, contractual partners may be free to choose their own base conditions.