This is used to randomize the initial model before producing each final model. If set to None then no randomization is done, and every model will be the same. You can set it to one of the functions in the randomize module — randomize.xyz (the default) to randomize all coordinates of standard residues, or randomize.dihedrals to randomize dihedral angles. (The amount of randomization carried out by these two functions can be tuned by changing AutoModel.deviation.) Finally, you could provide your own Python function; see the randomize module in modlib/modeller/automodel/randomize.py for examples to get you started.