Infield Fly called, ball drops, fielders run

[CecilOne]

Originally Posted by Bluefoot

The IFF rule does not 'assume' that the fly is caught. The batter is out, for the purpose of removing the force on the other runners on base, which would put the offense at a disadvantage. (where the def. could turn an easy double or triple play).
You won't find that IFF means the the def. caught the ball, because that's not correct.
If an IFF is caught, then any runners must tag up before advancing, just the same as any caught fly ball. But if an IFF is not caught, then runners can advance at their own risk, but aren't forced to do so, since the batter is out on the IFF and the force is removed.

IOW, an IF is the same as any other batted fly ball, except the batter is automatically out (unless foul) to eliminate the forces.
Tagging up is a separate issue, based on first touch if caught or if not caught, just like any other batted fly ball.