I understand that the IAT goal is to connect Florida, Pinhoti, and the trail that runs north from Katahdin to Cape Gaspe. That will accomplish the IAT running from Key West to Cape Gaspe.
The path from Key West to Cape Gaspe is known as the Eastern Continental Trail. The IAT covers everything north of Katahdin. This may help shed some light on things:
http://www.internationalat.org/Pages/SIAIAT_News/01167814-001D0211