Using covariates in the design is a nice, flexible tool to utilize. Sometimes I 'hack' the system in a way where there are 3 covariates available but in reality they are standard 'factors' from a DOE perspective. When I load them into Custom Design, I have the option to make either all 3 covariates as Hard or Easy to change. In some instances, I'd want more flexibility to make it a mixture of Hard and Easy to change variables (e.g., X1 is Hard, while X2 and X3 are Easy.)