As you search for an assignment of orders to trucks, you first try to assign the largest order, that is, the order with the greatest quantity. For a given order, you first try the "hardest" truck; that is, the one with the smallest domain; in other words, the one for which you have the fewest possible choices of orders that fit.

Here is the goal to choose the hardest truck to fill:

Truck ChooseTruck(IloSolver solver,
                   Order order,
                   IloInt nbTrucks, IloArray<Truck> trucks,
                   IlcBool aNewOne) {
  Truck best=0;
  IloInt eval = IlcIntMax;
  for(IloInt i=0; i < nbTrucks; i++) {
    IlcAnySetVar oVar = solver.getAnySetVar(trucks[i]->getOrders());
    IloInt cardMin = IlcCard(oVar).getMin();
    if (aNewOne) {
      if (cardMin > 0)
        continue;
    } else {
      if (cardMin == 0)
        continue;
      if (oVar.isRequired(order))
        return trucks[i];
    }
    if (! oVar.isPossible(order))
      continue;
    IloInt size = oVar.getPossibleSet().getSize();
    if (size < eval) {
      eval = size;
      best = trucks[i];
    }
  }
  return best;
}
 
ILCGOAL3(AssignOrder, Order, order, IloInt, nbTrucks, IloArray<Truck>, trucks) {
  IloSolver solver = getSolver();
  //Choose first an already used truck
  Truck t = ChooseTruck(solver, order, nbTrucks, trucks, IlcFalse);
  if (t == 0) {
    //Choose a new truck
    t = ChooseTruck(solver, order, nbTrucks, trucks, IlcTrue);
  }
  if (t == 0)
    fail();
 
  IlcAnySetVar tVar = solver.getAnySetVar(t->getOrders());
  if (tVar.isRequired(order))
    return AssignProducts(solver, order, t);
  else
    return IlcOr(IlcAnd(IlcMember(order, tVar),
                        AssignProducts(solver, order, t)),
                 IlcAnd(IlcNotMember(order, tVar),
                        this));
}
 
ILOCPGOALWRAPPER3(IloAssignOrder, solver, Order, ord, IloInt, nbTrucks, IloArray<Truck>, trucks) {
  return AssignOrder(solver, ord, nbTrucks, trucks);
}
 
IloGoal IloAssignOrders (IloEnv env, IloAnyArray orders, IloInt nbTrucks, IloArray<Truck> trucks) {
  IloGoal goal = IloGoalTrue(env);
  for(IloInt i = 0; i < orders.getSize(); i++)
    goal = goal && IloAssignOrder(env, (Order)orders[i], nbTrucks, trucks);
  return goal;
}
 
IloGoal IloEliminateTrucks (IloEnv env, IloInt nbTrucks, IloInt nbMax, IloArray<Truck> trucks) {
  IloGoal goal = IloGoalTrue(env);
  for(IloInt i = nbTrucks; i > nbMax; i--)
    goal = goal && IloAddConstraint(trucks[i-1]->getUsed() == 0);
  return goal;
}