The "untold" stuff is basically the western theater. It is a little misleading but that is the point. It talks about while the east gets all the love the war was actually won in the west.
The series was filmed by using footage from the new visitor center films at Shiloh, Chickamauga\Chattanooga, and Kennesaw Mountains. I am proud to have been a part of the filming of the last two and it has (in my opinion) great cinematic effects and visuals.