Freight class refers to a shipping method that is often used by businesses or individuals who need to mail large or bulky shipments. Most carriers follow the system of the National Motor Freight Traffic Association, which assigns a freight class to each type of shipment. A shipment is placed into any of 18 classes that are largely based on density. Other factors that impact the class of a shipment include handling characteristics, how much the shipment is worth, weight, the fragility of the items and distance traveled. The higher the number, the more expensive the shipping cost. Ping pong balls, which are light but lack density, are placed into the highest class, which is 500. On the other end of the scale, the lowest freight class is 50. A shipment of paper is scored as a 50 for a few main reasons: it is dense, it does not have much value and it is difficult to damage.