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What is Mill tolerance.

1) Is it the requirement of the code or the specific standard. Which one.?
2) Difference between Positive mill tolerance and negative mill tolerance?
3) In pipe wall thickness calculation , why positive mill tolerance value is added to the thickness at the end , why not negative mill tolerance .?
4) Why its value is fixed at 12.5% , is there any specific reason?
5) What are the list of products to whom this value is applicable?

Mill tolerance is the deviation allowed to a pipe manufacturer from the actual nominal thickness of a pipe.

1) This has been considered by relevant pipes ASTM standards, ASME B31 codes, and ASME B36.10 due to incapability of any mill to produce a pipe with an exact nominal thickness defined. The value in all the mentioned standards has been set to 12.5% of nominal thickness.
2) Both positive or negative mill tolerances can exist in a pipe. Positive would not be causing any significant impact but negative may put you in an odd situation if you didn't consider mill tolerance during piping thickness calculation. In addition to this, positive and negative mill tolerances are taken into account while performing stress analysis.
3) You would always like to have a thickness greater than the calculated pressure design thickness not anything below that.
4) This has been fixed by the applicable codes & standards as I have mentioned in point (1).
5) A-106, A-53 and some other piping materials which refers to A-530.

Tolerances exist for plates also. You may want to see ASTM A20 for getting some useful information of plate thickness under-tolerance.