Sitting volleyball (sometimes known as paralympic volleyball) is a form of volleyball for disabled athletes. In sitting volleyball, a 0.8 meter-wide net is set at 1.15 meters high for men and 1.05 meters high for women. The court is 10 x 6 meters with a 2-meter attack line. Players must have at least one buttock in contact with the floor whenever they make contact with the ball. It is also possible to block the serve and jousts are generally replayed.
Just like standing sports, sitting volleyball has regulations for it ball equipment and according to the World Organization Volleyball for Disabled Rulebook:
The ball shall be spherical, made of a flexible leather or synthetic leather case with a bladder inside made of rubber or a similar material. Its colour may be a uniform light colour, or a combination of colors. Synthetic leather material and colour combinations of balls used in WOVD, World Competitions and Zonal Championships should comply with WOVD standards. Its circumference is 65-67 cm and its weight is 260-280 g. Its inside pressure shall be 0.30 to 0.325 kg/cmÇ (4.26 to 4.61 psi) (294.3 to 318.82 mbar or hPa).
3.2 UNIFORMITY OF BALLS
All balls used in a match must have the same standards regarding circumference, weight, pressure, type, colour, etc.
WOVD, World Competitions and Zonal Championships, as well as National or League Championships, must be played with WOVD approved balls, unless by agreement of WOVD.
3.3 THREE-BALL SYSTEM
For WOVD World Competitions and Zonal Championships, three balls shall be used. In this case, six ball retrievers are stationed, one at each corner of the free zone and one behind each referee.
Official Sitting Volleyball Rulebook – http://www.sittingvolleyball.org/sittingvolleyballruleswovd.pdf
Image – http://www.rio2016.com/en/the-games/paralympic/sports/sitting-volleyball