Cell-free training balls.
Quality training balls without celluloid.
Since the introduction of poly balls a few years ago, a number of manufacturers are marketing training balls.
These are balls that are identical to their ITTF-approved 3-star cousins, with the possible exception that they are slightly less round, on average. Same weight, same hardness, same bounce.
Previously these balls were called 2-star or practice balls.
They are commonly used when players want to use lots of balls to practice, so their practice is more concentrated on actual play rather than picking up the ball every time a rally ends.
They also are used for multi-ball drills where the coach serves balls in a rapid-fire manner to the player who is practicing strokes, footwork, and/or other skills.