The field of play is not infinite. There is a wall and definitive boundaries marking territory in play. The poles mark the outer boundary of the fair field of play. After it has left the field of play the only way to determine if it was still in the fair field of play are the foul poles. The foul pole is a vertical extension of the foul line. A ball that hits the foul line is considered fair. As such a ball that hits any part of the foul pole should also be considered fair