Hitch classification depends on what is being towed and what tow vehicle is being used.
Ideally, a trailer should ride behind the tow vehicle as level as possible while towing. First of all, measure the distance from the ground to the bottom of the trailer coupler and from the ground to the inside top of your hitch opening, making sure that both truck and trailer are sitting level when measuring. The difference between the two measurements is the approximate drop or rise that you will need.
Since you may not be sure of your trailer height, you can follow these instructions. Again, measure from the ground to the center of the tow vehicle’s hitch opening and subtract that from 14″ since the average height for most trailers is 15″-17″ off the ground. Thus you will have the drop size you will need for most trailers on the market and will ensure the correct ride height in order for all axles of the trailer to stay in equal contact with the road at all times. For example, your vehicle measures 21.5″, then go with a 5″ drop.