(I recently answered this question on BGG Facebook page, here it is reprinted)
It is for a Tennis game by Richard Archer, patented as US20020171199 in 2001, now abandoned. The twenty-sided die would be used in the advanced game and also acted as the tennis ball. Here is a quote from the patent:
Referring to FIG. 2, in an alternative embodiment of the invention a special twenty-sided dice can be used for a separate advanced game. Note that W (winner) and O (out) are the same as on the basic dice shown in FIG. 1. However, instead of R (rally), this dice has letters telling the receiver of the throw how he must return the throw for the rally to continue. The markings on the dice telling the receiver how he must return the throw for the rally to continue are as follows:
(a) X (cross court)—receiver becomes thrower and must return dice cross court into the backcourt for the rally to continue. If it isn't cross court and in the back court (area behind the service court back line) thrower loses the point. If it is in cross court back court, read dice and continue the rally.
(b) ↓(down the line)—receiver must return dice down the side line and into back court for the rally to continue. If dice goes short, in front of service line, or cross court, thrower loses the point.
(c) OV (this has two meanings)—If it comes up on the serve it means the same as a let, serve it over. If it comes up on a rally, it means you must throw an overhead. From above the court, throw overhead overhand over the net and into the court. Remember if the overhead bounces completely out of the box you lose the point. Figure out how to throw it so this doesn't happen.
(d) L (lob)—must throw a lob (high arcing shot) over the net and into the backcourt for rally to continue. You lose the point on any lob that falls short, in front of the service court back line.
(e) D (drop shot)—must throw dice over the net and into service portion of court for rally to contine. A very short shot.