The ITF Rules of Tennis contain detailed procedures for inspecting ball marks resulting from close line calls that occur during officiated matches played on clay. One rule in this section can be invoked in acts of both good and bad sportsmanship.
If a player erases the ball mark before the chair umpire has made a final decision, they concede the call.
Appendix VI, Ball Mark Inspection Procedures, ITF Rules of Tennis, USTA Friend at Court
One way this rule could come into play is if a call is made and the player doesn’t want the umpire to inspect the mark. The player could prevent that by intentionally rubbing the mark out with their foot. In other situations where the mark cannot be located or cannot be conclusively read, the original call stands. This particular rule closes this loophole of unsportsmanlike behavior. A player who deliberately erases a mark automatically loses the point.
Fortunately, this rule is most frequently used as an act of good sportsmanship in actual matches. A player can rub out the mark as a signal to both the umpire and their opponent that the ball was good. That effectively concedes the point.
That simple act saves the umpire from having to climb in and out of the chair. In addition, it keeps the match moving along without interrupting the action. It’s just the nice thing to do.
If you pay close attention during clay court matches, you will see the players who grew up on that surface doing that all the time. It is one of the more subtle aspects of tennis etiquette.
It is good advice to any umpires out there, to always erase a ball mark after ruling on one, if you are called to inspect one.
This simple act will prevent ongoing discussions about the call, including discussions about the wrong mark.