The shaft of a bolt will have identification numbers or a code on it, to describe the bolt's physical properties. On an aluminum shaft the numbers will tell the arrow diameter and arrow wall thickness. The wall thickness determines the overall bolt weight. These numbers are used to describe the stiffness, or spine, of the bolt as well.
The carbon shafts also have numbers or coding to represent the varying weight and stiffness of the shaft. This is important as it will let you know how much flex is in the shaft. Your crossbow will require a shaft of the correct stiffness or spin to provide the performance and give you the accuracy required for hunting.
IMPORTANT! Always follow manufacturer's recommendations when determining the type of bolt to use with your crossbow!