In Tennis what are "practice balls"?

Answer 1

Practice Balls are Balls that did not make it through the quality control inspection process. Basically a reject ball, however the reason for rejection could be very minor such as the seems were too small, too big, or uneven. Other reasons which could affect play is that the felt is uneven or didn't make the specs, the core mold could not have uniformity meaning it could be light on one half from insufficient rubber composition. Normally practice balls are marked with a XXX or similar form of identification. However the #1 reason why a ballis deemed a practice ball is because it did not make ITF playing standards, or USTA approved play.