If the design contains process or amount variables, they must be held at a fixed level because these variables are not displayed on the plot.
You can set the hold level for the process variables at their low, middle, or high settings, or you can set a specific level for each. The hold values for process variables must be expressed in coded units.
For more information, go to What is the difference between coded units and uncoded units?.
If your mixture design contains text process variables, you can only set their holding values at one of the text levels.