The reinforcement rail is located in your sill, between sill and heater channel, and makes the car more rigid. It was initially designed for the Karmann Ghia type 14 convertible,  but all convertible need it, otherwise doors will not close nicely very soon after a restoration. But because we always want to improve our cars, we started to use these rails also in our coupés, with a nicer car and a longer life as a result… so you have to use them on your convertibles, and you can use in your coupés… that  is said…:)