Nine global parameters used in Pascal procedures that defines the values of X, Y, and Z .these parameters are available in explicit presentation to edit, entitled ‘Animation parameters’ .which it means these parameters can use to animate the surface in time.The scheme above shows the parameters used in Torus formulas.

Note 1: these parameters are disabled until you define animation parameter from explicit presentation.   

Note 2:  all parameters are available in global scope with the same name.

Note 3:changing the parameter in modify panel will force the program to regenerate the mesh.

Note 4: you can animate these parameters like other parameters in 3ds MAX.