Once you have completed filling in the Responses and Factors outlines, click the Continue button. This brings you to the next phase of design construction, where you either explicitly or implicitly choose an assumed model.
In Custom Design, when you click Continue after filling in the Responses and Factors, you see the Model outline. An example, for the design used in the Box Corrosion Split-Plot.jmp sample data table, is shown in Model Outline for Box Corrosion Split-Plot Experiment. The assumed model requires that the Furnace Temp and Coating main effects, and their interaction, be estimable. The design that is generated guarantees estimability of these effects.