Body interactions object is scoped to a body or multiple bodies while contact regions object is scoped to a face or multiple faces. By default, all the external faces of all bodies in an Explicit Dynamics model and their newly-created external faces after element erosion during simulation time will be automatically taken into account in contact calculation. In contact regions, only the scoped faces are participating contact calculation. Contact region objects have less contact calculations to conduct than body interactions object does. Thus it runs faster than body interactions but may miss the contacts especially if element erosion occurs.