something do not understand about code in polyhedra_ig2.cpp

hello
below is the code i am reading:
bool Ig2_Polyhedra_Polyhedra_ScGeom::go(const shared_ptr<Shape>& shape1, const shared_ptr<Shape>& shape2, const State& state1, const State& state2, const Vector3r& shift2, const bool& force, const shared_ptr<Interaction>& interaction) {
 const Se3r& se31=state1.se3;
 const Se3r& se32=state2.se3;
 shared_ptr<ScGeom> geom;
 bool isNew = !interaction->geom;
 if (isNew) {
  Ig2_Polyhedra_Polyhedra_PolyhedraGeom ppGeom = Ig2_Polyhedra_Polyhedra_PolyhedraGeom();
  ppGeom.interactionDetectionFactor = interactionDetectionFactor;
  bool pp = ppGeom.go(shape1,shape2,state2,state1,shift2,force,interaction);
  if (!pp) {
   return false;
  }
  shared_ptr<PolyhedraGeom> pGeom = YADE_PTR_CAST<PolyhedraGeom>(interaction->geom);
  geom = shared_ptr<ScGeom>(new ScGeom());
  geom->radius1 = (pGeom->contactPoint-se31.position).norm();
  geom->radius2 = (pGeom->contactPoint-se32.position+shift2).norm();
  interaction->geom=geom;
 } else {
  geom = YADE_PTR_CAST<ScGeom>(interaction->geom);
 }
 const Real& radius1 = geom->radius1;
 const Real& radius2 = geom->radius2;
 Vector3r normal=(se32.position+shift2)-se31.position;
 Real norm=normal.norm(); normal/=norm; // normal is unit vector now
 Real penetrationDepth=radius1+radius2-norm;
 geom->contactPoint=se31.position+(radius1-0.5*penetrationDepth)*normal;//0.5*(pt1+pt2);
 geom->penetrationDepth=penetrationDepth;
 scene = Omega::instance().getScene().get();
 geom->precompute(state1,state2,scene,interaction,normal,isNew,shift2,false);
 return true;
}
question1: what does the parameter shift2 do?
question2:what does se31.position refer to? i know that the position of sphere means the centre of it, but as to polyhedra, which point can define the position of it? Is the positon means the centroid of polyhedra or something else?
lookforward for your answer.