Calorific value of a gas in combustion
Calorific values are usually quoted under standard conditions making the distinctions a matter of preference and convenience

The apparently different units used in quoting heat of combustion are not as different as they appear. We need to know heat of combustion for many reasons (eg how to bill domestic customers for the correct amount of heating or cooking their gas supply delivers). But the units used are all equivalent once we define a standard set of conditions when measuring them. 

For example, a mole of pure methane at standard temperature and pressure (STP) occupies 22.4 L and will weigh about 16g and will burn in air releasing about 890kJ of energy. This means that, if we stick to the same conditions (STP in this case) we can interconvert the energy/mol, energy/mass and energy/volume with a simple calculation. 

Which one we choose to use is a matter of convenience. Domestic gas meters might choose the energy/volume as they measure volume flow at a standard pressure and temperature. Liquefied gas ships may choose to measure by mass as the methane is mostly liquefied and the mass may be easier to measure than the volume, temperature or pressure (but, knowing the mass, we can work out the volume at STP to work out how much domestic gas can be produced). If a chemist is working on some reaction scheme, then they will probably prefer molar metrics as they make balancing the equations easier.

But the essential point is that a given amount of substance always has the same heat of combustion. How you measure the amount is a matter of convenience.