I think this is the rule. I think they cited him on Unsportsmanlike Conduct, but I added Verbal Abuse, just to be helpful:
d) Verbal Abuse
i) Players shall not at any time directly or indirectly verbally abuse an official, opponent, sponsor, spectator or any other person within the precincts of the tournament site. Verbal abuse is defined as any statement about an official, opponent, sponsor, spectator or any other person that implies dishonesty or is derogatory, insulting or otherwise abusive.
ii) Violation of this section shall subject a player to a fine up to $20,000 for ATP Challenger Tour tournaments, $30,000 for ATP Tour 250 tournaments, $40,000 for ATP Tour 500 tournaments, $60,000 for ATP Tour Masters 1000 tournaments for each violation. The maximum fine will increase by one hundred percent (100%) for each consecutive violation during the same calendar year. In addition, if such violation occurs during a match, the player shall be penalized in accordance with the Point Penalty Schedule. In circumstances that are flagrant and particularly injurious to the success of a tournament, or are singularly egregious, the ATP Supervisor may refer the matter to the ATP Members Fines Committee who shall conduct an investigation to determine whether the player Major Offense of Aggravated Behavior or Conduct Contrary to the Integrity of the Game has occurred. Prize money earned at that event shall be held by ATP until the ATP Members Fines Committee has concluded their investigation and made a determination.
Audible Obscenity
- i) A player shall not use an audible obscenity while on-site. Audible obscenity is defined as the use of words commonly known and understood to be
profane and uttered clearly and loudly enough to be heard.
- ii) Violation of this section shall subject a player to a fine up to $5,000 for each violation. In addition, if such violation occurs during a match, the player shall be penalized in accordance with the Point Penalty Schedule. In circumstances that are flagrant and particularly injurious to the suc- cess of a tournament, or are singularly egregious, a single violation of this section shall also constitute the player Major Offense of Aggravated
Behavior.
Unsportsmanlike Conduct
- i) Players shall at all times conduct themselves in a sportsmanlike manner
and give due regard to the authority of officials and the rights of opponents, spectators and others. Unsportsmanlike conduct is defined as any misconduct by a player that is clearly abusive or detrimental to the success of a tournament, ATP and/or the Sport. In addition, unsportsmanlike conduct shall include, but not be limited to, the giving, making, issuing, authorizing or endorsing any public statement having, or designed to have, an effect prejudicial or detrimental to the best interest of the tourna- ment and/or the officiating thereof.
- ii) Players and their support team members accredited at any event must comply with the physical distancing and COVID-19 precautionary mea- sures applicable for each event. Any repetitive or blatant breach of those measures may be considered a violation of the code of conduct under this Section or the Major Offense of Conduct Contrary to the Integrity of the Game depending on the severity of the violation.
iii) Violation of this section shall subject a player to a fine up to $20,000 for ATP Challenger Tour tournaments, $30,000 for ATP Tour 250 tour- naments, $40,000 for ATP Tour 500 tournaments, $60,000 for ATP Tour
____________
Further down there is also this:
m) Default
i) During the match. The Supervisor may default a player either for a single violation of the Code (immediate default) or as outlined in the Point
Penalty Schedule.
So, it's clear enough what you can't do, in terms of verbal abuse, the point penalties and the fine schedule, and that there is the provision for the Supervisor to immediately default you, if they deem in egregious enough. However, when we read the rules, I think we can all think of flagrant examples of when players have been very abusive, at least to official, with nothing, or no more than a warning, at first offense. Or a code violation, which still offers no penalty, if you don't offend again. Note that it doesn't mention obscenity, but it does mention impugning the integrity of the official. Here, I'm particularly thinking of Medvedev. I'm sure he's done that more than once, but most recently at Wimbledon in the SF v. Alcaraz. They actually considered df-ing him, but I guess it was a Wimbledon SF, with top 10 players, so they didn't. As
@PhiEaglesfan712 suggested above...it doesn't seem to be evenly applied. As far as I can tell, Shapo dropped the F bomb with the spectator. Not the same if you drop it with an official, I guess. Then it's just a violation.
