Mounts on the door side and gives the lock body a flat surface to mount too. Can be modified. Shim must always be directly behind where lock rod exits at bottom. Even to lock body at top. Shim is white and may have surface and finish imperfections. Shim may be visible when used. Not always required for door sides with plastic plugs. Plugs can be removed and body mounted over. Check your door individually. Shim not included with each restraint model.