The first system is a system before we add our non volatile solute. The second system is after the addition of the non volatile solid. Before addition, a certain vapor pressure is created due to the molecules found in space above. After addition, some of the solid molecules will be replaced by the non volatile cellulose, and so less will evaporate, and therefore the vapor pressure will be less. Now, adding volatile cells or molecules that do evaporate increases vapor pressure according to a modified version of Rolls Law. And that's simply because now we have the solid molecules evaporating and the volatile solid molecules evaporating.