What are the differences between bolt strength grades 4.8, 8.8 and 12.9, and how to select them?

Grade 4.8: Low carbon steel, tensile strength ≥400MPa, yield strength ≥320MPa, for general light-load and non-critical connections. Grade 8.8: Medium carbon steel quenched and tempered, tensile strength ≥800MPa, yield strength ≥640MPa, for general heavy-load and main equipment. Grade 12.9: Alloy steel, tensile strength ≥1200MPa, yield strength ≥1080MPa, for high-strength, vibration/impact conditions (e.g., construction machinery, automotive chassis).