The body structure of the Audi A4 and Volkswagen Passat models is essentially a shell with bolted on components and sub-frames such as the front fenders, doors, hood, trunk lid, tailgate and the suspension crossmembers.
The rear fenders are actually an integral part of the shell and their replacement requires the use of specialized equipment to remove, properly position and weld in place. Furthermore, the Audi A4 and 1998-00 Passat models use a galvanizing process to protect their bodies against corrosion which must be considered when performing a structural body related repair.
The roof of the vehicle is also an integral part of the body shell and is supported by a series of boxed pillars that run from the floor pan the roof and provide strength to the roof, sides and overall structure of the vehicle. These pillars are referred to from front to rear as the "A", "B", "C", and the "D" pillars. The A-pillar for example, supports the windshield and is also the sealing surface for the forward portion of the front door.