This allows the degree of MD refinement of the models to be adjusted. You can define your own function for this purpose, set it to the special Python value None (in which case no additional refinement is done) or use one of the predefined functions in the refine module -- refine.very_fast, refine.fast, refine.slow, refine.very_slow or refine.slow_large -- which perform different amounts of MD annealing. See the modlib/modeller/automodel/refine.py file for more information.
See Section 2.2.2 for an example.