When a multi-mode differential viscoelastic model is selected, it is strongly advised that all modes obey the same constitutive equation. This requirement is even enforced for some models. Despite this, a multi-mode model may involve a long series of linear and nonlinear parameters, which need to be identified on the basis of a few data only. You have the option of specifying that nonlinear parameters of a given type will be assigned the same value for all modes. This may sometimes improve the fitting procedure by increasing the sensitivity of the model properties with respect to nonlinear parameters.

Using identical values for nonlinear parameters of a given type is not a requirement; by default, the nonlinear parameters are independent. It is interesting to note that the time-temperature equivalence is advocated for justifying such a choice, although that argument does not formally hold for nonlinear properties. See Empirical Rules and Principles for details.