Khatami (B) Attacked

Muhammad Khatami's brother, Muhammad Reza Khatami, was attacked while campaigning yesterday. Incidentally, could someone fill an apparent gap in my knowledge of the Iranian political system. I've seen Khatami described as Mustafa Mo'in's "running mate." What does that mean exactly? Iran has a system with a bunch of Vice Presidents for different things, kind of like an American corporation. Is there an official #2 that's announced during the campaign?