What happens to leftover campaign funds when a candidate drops out of the presidential race?

Robert Kelner is quoted in Fortune regarding the capabilities of Super PACs in a presidential election and what happens campaign funds when candidates drop out of the presidential race. Mr. Kelner says, “Super PACs have fairly broad discretion on what they can do with excess funds.” He adds, “Where you see a lot of money left over in the Super PAC after the candidate drops out, that will probably tell you something about how seriously the Super PAC took the race, to begin with."