Do I need an Under Rail or an Over Rail bed liner?

Under rail sits inside the vehicle load bed, against the side walls and bulkhead, with nothing going over the edge of the load area. This is the option if you are looking to fit any kind of load bed cover to your vehicle.

Over rail bed liners have a top edge on them, designed to go over the edges of the load bed and thus protecting the metal and paint work all around. The best option if you are keeping an open load bed.