Is it possible to upload expository papers to arXiv? I would like to know if arXiv allows authors to upload expository papers (for example, a paper in which the author explains a specific topic). If this is the case, how should those papers be written? And will they be automatically moved to the General Mathematics section?
The official line appears to be that:


  arXiv accepts only submissions in the form of an article that would be refereeable by a conventional publication venue. This excludes [...] reports that do not contain original or substantive research [...]


That said, I see expository and survey papers appear there all the time, and nobody seems to mind.  I think if your expository paper is of a quality that you would consider submitting to an appropriate journal (there are many that publish expository papers) or lecture notes series, then it's fine for arXiv.  If it's just your course notes on the fundamental theorem of calculus, don't do it.

I don't see why they should be moved to "General Mathematics"; they should be categorized with the topic that the paper is about.