It used to go to the destination listed on the tag and then it would be re-routed from there to your new destination, if that was closer to the delivery address you gave the baggage office. But now it seems that the bag gets rerouted most times that I do. A few months ago, my flight ex-PHL was delayed extensively due to weather. The ExPlat desk was able to re-route me to a nearby airport. The agent said that I needed to check-in ASAP for the new flight as this would trigger the note to the PHL baggage handlers that the bags needed to be re-routed. Never heard that before and not sure if it's true, but an interesting point.