Some models are linear in the parameters (for example, a quadratic or other polynomial); others can be transformed to be such (for example, when you use a log transformation of x). The Fit Model or Fit Y by X platforms are more appropriate in these situations. For example, in the Fit Model chapter of Fitting Linear Models, there was a significant linear relationship between oxygen uptake and time spent running. For more information about Fit Model, see Fitting Linear Models. For more information about Fit Y by X, see the Basic Analysis book.
The Nonlinear platform is the place to fit models that are nonlinear in the parameters. This chapter shows the analysis of a nonlinear relationship: drug toxicity as a function of concentration. The effect of concentration on toxicity changes from low to high doses, so this relationship is nonlinear.