I think you are trying to find functional difference between Moist air and Gas library of Simscape. Moist air mixture has additional Trace gas which is an optional third species in addition to dry air and water vapor. The usable components while modelling gas models are Gas mass flow rate, pressure source blocks and in case of moist gas models, it includes moist air mass flow rate, pressure, volumetric flow rate source blocks. For more information, you can refer to the documents of gas systems and moist air systems.
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