title: What's the purpose of the noncharacters U+FDD0 to U+FDEF?
body: <p>U+FFFE needs to be a noncharacter in order to allow the Byte Order Mark to work.</p>
  
  <p>U+FFFF is described in <a href="http://www.unicode.org/versions/Unicode6.0.0/ch16.pdf" rel="noreferrer">The Unicode Standard</a> as "useful for internal purposes as sentinels".  Makes sense.</p>
  
  <p>But I can't figure out, and The Unicode Standard doesn't really explain, why the set of noncharacters includes some random block within "Arabic Presentation Forms-A".  What are these <em>for</em>?  (Besides <a href="http://xkcd.com/380/" rel="noreferrer">the eye of the basilisk</a>?)</p>