bolts normally come in hex head, flat Head and other fancy bit configurations and are use to hold the bow together.
Arrows come with flat or 1/2 moon knocks. If your bow uses flat knocks then you should use flat knocks. however you can use 1/2 moon knocks on bows designed for the flat knocks but you should not use flat knocks on bows designed for 1/2 moon knocks. it has to do with the way the string is released and how it hits the arrow. if it is released from under the arrow you have to use a 1/2 moon knock to keep the string from ridding over the arrow, If you string is released from the top then the string will hit the bottom of the arrows and less chance to ride up over the arrow that is why the flat knock works. The flat knocks are better because you do not have to worry about how the arrow is stetting on the bow as long as you have a feather in the channel you are good to go.
