Once you have assigned an order to a truck, you distribute the products demanded by that order among the tanks on that truck. For a given product, you choose the "hardest" tank; that is, the one for which you have the fewest possible choices of products; in other words, the tank with the smallest domain.

The goal to choose the hardest tank to fill looks like this:

Tank ChooseTank(IloSolver solver, Product p, IlcAnySet tankset) {
  Tank best=0;
  IloInt eval = IlcIntMax;
  for(IlcAnySetIterator it(tankset); it.ok(); ++it) {
    Tank tank = (Tank)(*it);
    IlcAnySetVar pvar = solver.getAnySetVar(tank->getProductOrders());
    if (pvar.isRequired(p))
      return tank;
    if (! pvar.isPossible(p))
      continue;
 
    IloInt size = pvar.getPossibleSet().getSize();
    if (size < eval) {
      eval = size;
      best = tank;
    }
  }
  return best;
}
 
ILCGOAL2(AssignProduct, Product, product, Truck, truck) {
  IloSolver solver = getSolver();
  //Choose an already used tank
  IlcAnySetVar stanks = solver.getAnySetVar(truck->getTankVar());
  Tank tank = ChooseTank(solver, product, stanks.getRequiredSet());
  if (tank == 0)
    //Choose a new tank
    tank = ChooseTank(solver, product, stanks.getPossibleSet());
  if (tank == 0)
    fail();
 
  IlcAnySetVar stankvar = solver.getAnySetVar(tank->getProductOrders());
  if (stankvar.isRequired(product))
    return 0;
  return IlcOr(IlcMember(product, stankvar),
               IlcAnd(IlcNotMember(product, stankvar),
                      this));
}
 
ILCGOAL2(AssignProducts, Order, order, Truck, truck) {
  IloSolver solver = getSolver();
  IlcAnySetVar prodVar = solver.getAnySetVar(order->getProducts());
  IlcGoal g = IlcGoalTrue(solver);
  for(IlcAnySetIterator it(prodVar.getRequiredSet()); it.ok(); ++it)
    g = IloAndGoal(solver, g,  AssignProduct(solver, (Product)(*it), truck));
  return g;
}