This is the order sent out by the government of Japan at Neo-Tokyo-2 in "Air", ep.25. The order means the "abolishment of the special legal protection towards the intelligence organsization NERV, and the transfer of its command to the government of Japan".
Since "A-17" was issued when the 8th Angel was captured, there has been other "A-" orders. They were all emergency action orders related to NERV.