When buying system windows, focus on core parameters and authoritative certifications. Parameters: profile wall thickness ≥ 1.4 mm (≥ 2.0 mm at load-bearing parts is better); prefer 6063-T5 aluminium; choose PA66 nylon thermal-break strips (check the section marking), not PVC, for reliable insulation; hardware from German brands such as Hoppe or Siegenia with 100,000+ cycles; at least 3 sealing lines with EPDM gaskets; glass 5 mm + 20A + 5 mm tempered insulating, with Low-E in the north and tempered certification for high floors. Certifications: GB/T 8478-2020 (Aluminium Alloy Windows and Doors), CE for the EU, NFRC for North America, and a complete system certification such as German PHI to ensure the components are matched and long-tested — avoiding 'fake system windows'.