These details are supplied by the Gas Supplier.
The heating value is used to calculate the amount of mega joules (Mj) in natural gas. The heating value is impacted by the temperature and pressure at different geographical locations.
The pressure factor, also provided by your local distribution company, is used to calculate your gas consumption. The pressure and speed of the natural gas delivered to your premises will often vary and is based on environmental factors, such as temperature and the appliance that is consuming the gas.
Hope this helps.