Regular (aka Championship level) classes were the first type of classes that AKC offered. In 2002, AKC also started offering PREFERRED level classes. These classes allow a dog to jump ONE jump height lower than measured AND gives another 5 seconds added onto the Standard Course Time.
For example, a dog that measures 22.25 would normally jump in the 24″ height division. They may enter the Preferred level class in the 20″ height division. Dogs may earn titles in each type -Championship and Preferred.
You may enter Preferred and Championship during the same event weekend but not for the same class on the same day. For example, you may enter Novice Preferred Standard and Novice Regular JWW but you may not enter Novice Preferred Standard and Novice Regular Standard.
You may switch back and forth between Preferred and Regular heights for different events. You may ONLY enter the correct Preferred Jump Height. Unlike the Championship class, you may NOT enter a higher level Preferred Jump Height.
You may move laterally from the Regular classes to the Preferred classes, without starting over in Novice Preferred.  For example, if your dog is in Masters 20″, you could move laterally to Masters Preferred 16″.  You may not move laterally from Preferred to Regular height. Please refer to the AKC Agility Rules and Regulations page 59 for specific details.