They cite RFC6920 [1], which uses the "host" portion of the URL as an optional "authority" field.

[1] http://tools.ietf.org/html/rfc6920#section-3

Because the ni scheme is separate from this proposal and it supports having a server there if you retrieve named information from somewhere else (implementation defined i think).

Could be

  ni:
  //
  <empty field>
  /

file:: URIs can have it, too (http://superuser.com/questions/352133/what-is-the-reason-tha...)