When I calculate the additional scalar, I need to define the concentration of the scalar at the boundaries. But what is the dimenstion of the concentaration? In the manual, it is defined as kg/kg. Is it (kg/kmol)/(kg/kmol) or (kg/m3)/(kg/m3), or something else?

Thanks in advance!

The solved concentration is mass fraction so the unit is kg/kg: mass fraction in a cell is defined as Mass of a scalar/total mass.