What is included in the Tow Package for the Expedition

The LeRoys wrote:
I'm looking at the newer Ford Expedition & Expedition EL models and looking at their tow ratings on the guide on this site. There is a significant difference in two capacity if they have the "tow package". However, I haven't been able to find a definition of what is included in that package. Is it something that can be added if I purchase a used vehicle?