In 3D Fill or 3D Fill + Pack analyses run by the mhb3d solver, you can supply a function that will be called whenever the solver wants to know the viscosity of the melt at a given temperature, shear rate and pressure. You can also supply a function that will calculate the viscosity of the melt at a given node in the model, or at a given laminate of a beam node in the model.
If you supply them, the solver will call an initialization function at the start of the solve and a cleanup function at the end of the solve, in case your viscosity function needs to allocate memory or open a file.
You can supply a configuration string in the "Parameter Text" field of the advanced solver parameters, which will be made available in a utility function. You are invited to put coefficients for your viscosity model, or the name of a file containing additional configuration, into this string.
If your user function needs to know a solver parameter or the position of a node in the model, it can call back into the solver to get this information. These functions are listed in the Solver API Reference document, which you can access from the Help drop-down menu on the top right-hand of the user interface. Click ? then select Solver API Reference.