Points to note when buying a system window. Brand and certification: choose a well-known brand and check whether the product has passed ISO, CE and similar certifications. Performance parameters: pay attention to air tightness, water tightness, wind-pressure resistance and other indicators. Hardware: quality hardware (such as German HOPO and SIEGENIA) extends service life. Installation service: a professional installation team ensures the door and window perform at their best. After-sales service: comprehensive after-sales support removes worries.