Tennis Balls, the differences
Most tennis balls come in two versions - regular duty and extra duty. Like shoes, the type of court you play you determines the type of balls you should use.
If you play on a hard court surface, you need Extra Duty tennis balls. Due to the unforgiving nature of those courts you need a ball that will wear down less quickly. Extra duty balls have a thicker felt, that is woven not as tightly around the ball's core. This means the felt will perform better, for longer.
If you play on a clay court surface, you need Regular Duty tennis balls. Since the clay may enter under the felt you need a ball that will minimize that from happening. Regular duty tennis balls have a thinner felt that is woven tightly around the ball's core. This means that clay cannot penetrate the felt and will not cause the ball to puff up, preserving its integrity over time.
If you use a hard court, extra duty ball on a clay court surface, the clay will penetrate the felt and cause the ball to puff up. If you use a clay court, regular duty ball on a hard court surface, the ball's durability is compromised and it will wear down much quicker.
Learn more about junior progressive tennis balls here.