For every .B model file produced during modeling, a corresponding .D optimization trace file is produced, using the actions.trace() periodic action. (When loop modeling with loopmodel, a .DL file is produced for each .BL loop model.) These files contain information about the progress of optimization, such as the iteration step, atomic shifts in space, energies and gradients. By default, this is written out every 10 steps, but you can change the frequency by assigning to this variable, or turn it off completely by setting it to zero.
See also automodel.get_optimize_actions() and automodel.get_refine_actions().