What do standard thread tolerances 6g, 6H and 5g6g mean, and how to use them?

6g: Commonly used for external threads (bolts/screws), medium precision, general assembly. 6H: Commonly used for internal threads (nuts), medium precision, matches 6g external threads. 5g6g: High-precision combination for external threads (pitch diameter 5g, major diameter 6g), used in scenarios with high precision and anti-loosening requirements.