In sensitivity analysis, independent variables should be the property which is writable (i.e used as an input during calculation routine).

In the simulation of rigorous distillation column, reflux ratio cannot be entered directly as a property. It is one of the specification types listed under condenser specification and therefore it can only be entered as a part of condenser specification value. So if we want to optimize the reflux ratio, it has to be given as input through condenser specification value while running the simulation. As condenser specification value is writable and reflux ratio is selected as the specification type, that's why we need to choose condenser specification value under independent variables while doing sensitivity analysis.

If some other specification type is chosen as condenser specification (other than reflux ratio) like product molar flow or heat load, then it's not the reflux ratio that gets optimized but the property which is selected as condenser specification will be optimized.