There's no limit to how many times a player can ask a chair umpire to check out a mark but it doesn't mean the chair umpire is obliged to get out of the chair.
We felt that we had to pursue every means possible to utilize technology to make sure that calls were accurate without losing the human element of officials on court.
We all have seen matches turn on questionable calls. With all that's on the line in tennis these days, we felt we had to pursue every means possible to utilize technology to make sure that calls were accurate.