Hex Nuts are available in many thread types; both metric coarse (standard) and metric fine threads, Unified National Coarse (UNC), Unified National Fine (UNF) and British Standard Whitworth (BSW).
Metric sizes are made to the Din 934 standard. They also come in both 316 (also known as A4) and 304 (also known as A2 or 18-8) grades with different tensile strength options.
The nut is normally stamped its tensile strength and grade e.g. A2-50 or A4-70 where the A2 refers to 304 grade and A4 refers to 316 grade the 50 refers to the tensile strength of 500 Mpa and the 70 refers to 700 Mpa.
However stronger Bumax 88 and A4-80 Nuts are also available made to 800MPA Tensile Strength. As are Bumax 109 Hex Nuts (High Strength Stainless) made to Class 1000 Mpa Tensile Strength.