Pro hac vice (IPA: [pr?? hæk 'vi:t?ei]), Latin: "for this occasion" or "for this event" (literally, "for this turn"),^[1] is a legal term usually referring to a lawyer who has not been admitted to practice in a certain jurisdiction but has been allowed to participate in a particular case in that jurisdiction.  -wikipedia

Every State in the U.S. has their own licensing, and self regulation scheme.  Lawyers set up the rules to benefit their industry, but even in such a peculiar notion of allowing only lawyers to police themselves, there are limits.  Thus, a lawyer registered to practise law in California can not automatically practise law in any other state, like Delaware, New York, or Michigan.  Each state crafts their own rules for allowing a foreign lawyer to appear, under pro hac vice.  Thankfully, this process opens up the door to allow any given state to apply their local state professional ethics and conflict of interest laws to the visitng lawyers.  In fact, pro hac vice allows each state a safety net to filter out dirty lawyers, and even prosecute them.

I don't know,
  but I've been told
Pro Hac Vice
  is a double edged sword
       - Led Zeppelin