A mixture material is a material with different fluid components which are mixed on a molecular level. The default fluid material in Fluent is Air, when activating “Species Transport” in the “Models” task page the default mixture material can be seen under “Materials”. “Mixture-template” is the mixture material equivalent to Air which is composed of nitrogen, oxygen and water-vapor. If you open it using Create/Edit you can add or remove species. In this way you can also create your own mixture materials by first adding the fluid materials from which you want to build your mixture. The most abundant species should be put as the last species in the list (at the bottom of the list). In case of having N species in the mixture list, Fluent only requires to solve the species transport equations on the first N-1 species. The mass fraction of the Nth (the last) species is calculated with the constraint on the summation of mass fractions of all N species being equal to 1.0.