What's the difference between "extra duty" and "regular" felt?

Answer 1

Extra duty balls are more durable and are meant for play on hard courts which are tougher on balls than other surfaces.

Regular felt balls play better on clay courts because they have less nap, thereby allowing less clay to get clumped in the felt and potentially slowing down the flight of the ball.

Answer 2

The Red print tennis balls are Regular duty felt, and the Black print tennis balls are Extra Duty Felt. This has been used by Penn Tennis balls for a number of years. Some people prefer one ball vs. another on different playing surfaces such as fast hard court, slow hard court, Red Clay, Green Clay, Grass..etc. However the print refers to the thickness and amount of felt making up the tennis ball. Sometimes players with flat strokes or a serve and volley player will prefer the red number balls becuase it makes it more difficult for his opponent that may be a topspin player because he will have less felt to grab onto for his shots.