That depends on the event. I have seen all the following requirements for the breaking location (all behind the head string):
• Anywhere
• Within a 2-diamonds-wide central box
• Within an 18-inch-wide central box
• Within a 12-inch-wide central box
• Outside a 2-diamonds-wide central box (i.e., within 1 diamond of either side rail)
• Outside a 3-diamonds-wide central box (i.e., within half a diamond of either side rail)