I've noticed in the WC Official Match Results that there is a 5th official listed. I don't remember ever noticing a 5th official before. What are the responsibilities of the 5th official?
remember at this level a ref is either a CR or an AR, he is not both. The 4th will replace a CR when needed, the 5th replaces an AR when needed.
But there aren't any duties specifically assigned to the 5th official? I don't know what they would be.
There are none. FIFA took 5 three-man crews to this World Cup to essentially work as back-ups. The 5 referees have been assigned to work as 4th officials. That leaves 10 ARs, with nothing to do other than wait for their colleagues to get injured. So FIFA, smartly, assigned a 5th official for each game, just in case someone gets injured.