Select Use the restricted form of the model if you want to use the restricted form of the model instead of the unrestricted form.
A mixed model is one with both fixed and random factors. There are two forms of this model: the restricted model requires the crossed, mixed terms to sum to zero over subscripts corresponding to fixed effects and the other model does not. Many textbooks use the restricted model. Most statistics programs use the unrestricted model. Minitab fits the unrestricted model by default, but you can choose to fit the restricted form. The reasons to choose one form of the model instead of the other are not well defined in the statistical literature1.
Your choice of model form does not affect the sums of squares, degrees of freedom, mean squares, or marginal and cell means. It does affect the expected mean squares, error terms for F-tests, and the estimated variance components.