I don't believe that all questions that are mathematical problems should have the ArXiv two-letter prefixes.  The right way of interpreting [nt.number-theory] is that it raises some issue of interest to number theorists.  Any question that nearly all final-year undergraduate mathematicians could answer does not deserve these tags.

Example: [Why is the decimal representation of 1/7 “cyclical”?][1] has the [nt.number-theory] tag: wouldn't it make more sense if it was tagged [arithmetic] instead?  What has this question to do with the distinctive kind of mathematics that number theorists do?  I would not expect to see such a question treated in an undregraduate course entitled "Number Theory".

I answered basically this question before, [MathOverflow tags + others ?][2].


  [1]: http://math.stackexchange.com/questions/443/why-is-the-decimal-representation-of-1-7-cyclical
  [2]: http://meta.math.stackexchange.com/questions/59/mathoverflow-tags-others/87#87