1. A method for managing behavior of a CAD model, comprising: receiving a geometric object and corresponding physical object, for the CAD model, in a data processing system; executing function code in a behavior object by the data processing system, to determine required references of the behavior object; receiving and storing an assignment of at least one of the required references of the behavior object to the physical object by the data processing system; storing the geometric object, physical object, and behavior object in the data processing system as associated with the CAD model; simulating operation of at least part of the CAD model by the data processing system, including executing the behavior object by the data processing system to modify the state of the physical object.