If a batter pops up or flies out down a base line, and the fielder catching the ball is within a couple feet of the foul line, the umpire will always signal the out, and then "fair" or "foul" depending on where the ball is caught.
Why is this? Runners can tag up on either a foul or fair ball. A missed play would likely be scored an error on the fielder regardless of which side of the line the ball dropped.
I know there's a (slightly) different scoring notation for caught fair and foul balls. If the left-fielder catches the ball in fair territory it's scored ("7" or "F7"); in foul territory ("7F" or "F7F"). Is there any reason to indicate fair or foul here other than as a courtesy to the scorer to inform him that he needs to write an extra letter?