20.E.2Referee-Called Replays
If the referee is unsure whether a serve met the requirements, they can call a replay. They have to do it before the return is hit.
Inside this rule
20.E.2.aReplay When Ball Release Is Not Visible20.E.2.bReplay for Doubtful Release20.E.2.cReplay for Doubtful Spin or Manipulation20.E.2.dReplay for Doubtful Foot Contact in the Serving Area20.E.2.eReplay for Doubtful Foot Contact Outside the Serving Area20.E.2.fReplay for Doubtful Upward Motion20.E.2.gReplay for Doubtful Paddle Head Height20.E.2.hReplay for Doubtful Ball Height20.E.2.iReplay for Doubtful Force on a Drop Serve