The only subjective element is ruling "ordinary effort" by an infielder. As for fair or foul, the rule is automatically void on a foul ball, although the umpir

The only subjective element is ruling "ordinary effort" by an infielder. As for fair or foul, the rule is automatically void on a foul ball, although the umpire should explicitly call "infield fly if fair" on a fly that may land foul. I literally saved my team's season one year when I wrote an upheld protest of a misapplication of the IFR. We won the replay, that got us into the playoffs, and we won the championship.