A heavy hex nut is designed to go onto a bolt with matching threads. They are called heavy because they are thicker in most dimensions than a standard finished nut. While often double chamfered and countersunk, they are frequently only single chamfered and countersunk, especially in larger sizes. The largest readily available one is 4" in diameter, but 6" diameter is not uncommon. Most structural nuts are heavy. Heavy does not refer to the grade.