A D is made of high-strength polymer.The buckle leaves a reduced IR trail. This is a lighter and less expensive option to traditional metal carbines.It's not the source of disturbing noises, like the metal ones, and it doesn't make scratches.It locks with a taut, rigid tongue into an opposing groove. It's only designed to attach things to the gear.It has a carrying capacity of 200 pounds, or about 90 kg, but it must not be used for climbing or similar activities where life would depend on the buckle!