Main roles of lawn tennis officials

The referee is the judge on all questions of tennis rules. The referee is normally one person for the tournament, so would not be present on court and has to be called if there is a decision to be made. The players have the right to call for the referee if there is any dispute on a question of rules.
The chair umpire, if there is one, is the judge of all questions of fact (unless it is a team competition and the referee is on court, in which case the referee also has final say on questions of fact). The players have no right of appeal on a question of fact except for appealing line calls to the chair umpire. The chair umpire also calls the score and marks the scorecard or electronic scoring system, and is the sole judge on ‘code of conduct’ matters on court, except that players may appeal to the referee in cases of default for exceptionally severe code of conduct violations. The referee may overturn the default, but not the chair umpire’s imposition of a penalty on the point penalty scale.
The line umpires and net umpires, if there are any,
only make calls about their own line or the net. They
may call the ball out or call a foot fault, or a net
touch from a service, but may be overruled by the
chair umpire. They are strictly not allowed to get
involved in anything else. They may if they wish
make a sign that a ball is in to indicate to the chair
umpire that they had good sight of the ball if it is
close, but they do not have to, or make a sign that
they were unsighted.
Line umpire’s may also report a code violation to the
chair umpire if they saw or heard something that
they think the chair umpire missed. The chair umpire
may or may not accept their version of the facts, and
may ask a player for their version, before making a
decision.

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