First off Saddleseat would be for a gaited horse like a Saddlebred, NSH etc.
Road Hack is judged at the walk, trot, canter, hand gallop. You usually ride with a light contact and its judged mainly on performance but also on conformation, and manners.
The fact the class is Open means that you'll be competing against Juniors, Adult Amateurs and Trainers. As well as possible Green Horses and old campaigners.
Gordon Wright once told a student to take up swimming because he would never be a good rider, that rider was George Morris.