Design of Experiment (DOE) enables you to see the effect of some intervention on the quality of the part. The input variables are the interventions. For example, you can change the melt temperature, or coolant inlet temperature, and see the effect on part quality.
The left hand panel of the DOE Builder Variables tab lists the input variables that you can select to study. The right hand side provides you with an opportunity to change the experimental range for each variable. Select each input variable that you are interested in, accept the default values or change them if you prefer, then click Next.