As per the ATP press release:
For the first time in men’s professional tennis, Video Review will be used by officials at the Next Gen ATP Finals, the ATP’s ground-breaking season-ending event for the world’s best 21-and-under players, set to take place in Milan, Italy, from 6-10 November.
The Video Review, delivered using advanced Hawk-Eye technology, will provide opportunities for players to challenge any judgement calls from the Chair Umpire such as Not-Ups, Foul Shots, Touches, or Invasion*.
*Examples of incidents that would be subject to Video Review at the Next Gen ATP Finals are:
• Not-Ups – double bounces
• Foul Shots – for example, deliberate double hits or carry; or hitting the ball before it has passed the net; the ball, prior to bouncing, hits a permanent fixture; or the racquet is not in the player's hand when touched by the ball.
• Touches – ball skimming racquet, clothing or body; or if a player, or anything he is wearing or carrying, touches the net, net posts/singles sticks while the ball is still in play.
• Invasion – player, or anything he is wearing or carrying, touches the opponent's side of the court while the ball is in play.
This move paves the ways for players to challenge more umpire decisions than whether a ball is in or out. Remains to be seen exactly how it will be implemented on the main tour if it's a success in Milan. Seems like it may be a bit more challenging technically and logistically than relying just on HawkEye itself, especially at smaller courts or tournaments that don't have extensive camera coverage.The Hawk-Eye technology behind the Video Review will use video feeds from all television cameras so that the Video Review operator can quickly search footage to find the correct angle for the decision to be made. The relevant footage will be sent to the Chair Umpire’s tablet who will review the video and decide whether to uphold or overturn the original call. All relevant footage will be played out to the in-house spectators on large video boards in real time, as well as on broadcast, to take the audience even closer to the action. There will be no limit to the number of Video Review challenges a player is able to make.
Future use of Video Review could include decisions on whether to award the point or replay the point in the case of a corrected out to good call by an official. In Milan, however, such instances will not arise as all calls made by Hawk-Eye Live are final.