In Creo 2.0 when set to automatic, it creates constraints based on what geometry you choose and their relative locations and values of thresholds set in the config settings. For instance, when you mate two surfaces, their original position (just before the final geometry selection) with respect to each other determine whether it is normal, coincident, distance, or angle.
There are several config settings that establish a threshold for how Creo selects each one. If the surfaces are coincident within one threshold but too far apart to be within another threshold then it selects distance, etc. If you know what these thresholds (epsilons) are you can reposition (ctl+alt+mouse, or use the 3D dragger) your component prior to selecting geometry such that it automatically selects what constraints you want.
comp_normal_offset_eps
comp_angle_offset_eps
auto_constr_offset_tolerance
auto_const_always_use_offset
Furthermore, by adjusting the thresholds with a few config options, you can effectively disable some of the constraints. If you always want coincident then you can adjust the thresholds/epsilons and a few config options to do this. These below have worked for me to make something always default to coincident unless I otherwise choose. Hope that helps.
auto_const_always_use_offset NEVER
comp_angle_offset_eps 91
comp_normal_offset_eps -1
