Class 1 threads are a loose tolerance. Class 2 threads comprise 90% of stainless fasteners and are normal commercial tolerance. Class 3 threads have a stricter tolerance and tighter fit such as socket cap and set screws. No definite relationship exists between tensile strength and tightness or looseness of fit. The symbol “A” added to threads, such as 2A, means external threads (screws), and “B” means internal (nuts).
With the exception of 10/32 diameter, which is extremely popular, coarse thread comprises 90-95% of hex head cap screws and hex nuts sold in 18-8 stainless, and perhaps 98% of other stainless items including machine screws and socket products. Coarse threads are deeper than fine threads with fewer threads per inch, so coarse threads may have greater protection against thread stripping, better tap in brittle materials, and better fatigue resistance, while fine threads may have better fit in thin-walled materials, higher torque strength, and increased tightness during vibration.

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