If you have aspirations to making it to the FIFA panel as a referee or assistant referee, or you have just been curious about the new Additional Assistant Referee (AAR) mechanics, please check out the video at this link:
Besides getting to hear Pierluigi Collina talk about refereeing, you can see how the AARs communicate with the referee. I was fascinated by the nature of the chatter on the radio. The video also notes that the AARs are actually staffed by members of the referee panel and the 4th official (previously always a referee) can be staffed by an assistant referee. This puts at least three referee-level officials on each match.