https://www.mathworks.com/help/ident/ref/polynomialregressor.html
polynomialRegressor - Specify polynomial regressor for nonlinear ARX model - MATLAB
Polynomial regressors are polynomials that are composed of delayed input and output variables.
nonlinear arxspecifypolynomialregressormodel
https://www.mathworks.com/help/ident/ug/validating-nonlinear-arx-models.html
Validate Nonlinear ARX Models - MATLAB & Simulink
Plot model nonlinearities, analyze residuals, and simulate and predict model output.
nonlinear arxvalidatemodelsmatlabsimulink