The ITF Court Pace Classification Programme has been developed to assist purchasers of tennis surface products to determine the type and speed of surface most suited to their requirements. The ITF classifies surfaces into one of five categories according to its Court Pace Rating: 1 - Slow, 2 - Medium-slow, 3 - Medium, 4 - Medium-fast, 5 - Fast. ...
https://www.itftennis.com/en/about-us/tennis-tech/classified-surfaces