What are a line umpire's duties, and when can the chair umpire overrule a call?

Line umpire duties:

• Rule on all balls on or near their assigned line. The decision is final unless, in the chair umpire's opinion, a clear mistake was made; the chair may then reverse the call or have a let played.
• Signal immediately to the chair umpire (unsighted signal) when unable to make a call.
• Correct an erroneous "Out" call immediately. If they realise they gave a "safe" signal on a ball that was out, they make the correct call immediately. If an "out" ball was not called out immediately, they stay silent.
• Call foot faults on their line (or its imaginary extension) after the serve is struck, and tell a player what caused a foot fault call if asked.
• Report to the chair any Code of Conduct violations they see or hear that the chair did not.
• A line umpire never works in a match that has no chair umpire.

Overrules (Appendix VII cases):

• The chair may overrule only when sure a clear mistake was made, and only immediately after the mistake, not at the end of the point.
• The chair must never overrule as a result of a player's protest or appeal.
• If the chair is unsure but thinks the ball was in, the chair may not overrule.
• A line umpire may correct a call after the score is announced, provided it is not because of a player's protest.
• If an "Out" call is corrected to good, the chair decides whether the original call was a hindrance: if so, replay the point; if not, the player who hit the ball wins the point.