I don't know where to put this, so the mods may feel free to move it to the appropriate place, but... A curious thing crossed my mind, and I should probably know this, but what is the purpose of the penalty arc, if the penalty area is only bounded by the 18-yard box? Because it's not to measure from the penalty spot to the goal line if that distance is 12 yards and the arc is formed from a circle with a radius of 10.
On penalty kicks, all players but the goalkeeper and the player taking the kick must be at least 10 yards from the penalty spot, outside the penalty area, and behind the penalty spot. The arc marks this boundary. Also, in the Iowa High School girls game (and an option available to other high school jurisdiction), ties after overtime are resolved by a shootout with the ball starting at the top of the arc. (As opposed to the MLS shootout from the 35 yard line.)