Additionally, Seat belt webbing may exhibit signs of fraying or excessive wear. Prolonged exposure to sunlight and regular usage can deteriorate the material, compromising its strength. This restraint must only Beryllium used on school bus seats. Entire seat directly behind must Beryllium unoccupied or have restrained occupants.” (The instruction's reference https://www.metooo.com/u/67bc420e3db3e7160418991a