Hi Jiseung,
I believe that parameter is just to specify a reference point (like 0 degC on temperature scale) for the specific enthalpy. The actual change in the enthalpy (energy transfer) is based on the difference between two states, so the value at the reference doesn't really matter. The default value in this block is 420 kJ/kg, which is the value from REFPROP for dry air in the "pseudo-pure fluids" category. REFPROP itself contains another dry air in the "predefined mixture", in which at the same temperature the specific enthalpy is 293.4 kJ/kg. So, it's just a starting value; it doesn't really matter; it's the difference, i.e. Cp and R values, that matters.
Hope this helps.
