According to DOD:
The game Squad Leader gives a very good demonstration of the difference between spotting rounds, harassing fire and fire for effect. First the Forward Observer (FO) establishes contact with the artillery unit. Then a spotting round is called for. Spotting rounds are then "walked" on to the target. When the spotting round is either on the target or the necessary adjustment is small enough to be within allowable limits, the FO calls for a Fire Mission, often with the phrase, "Fire for Effect." If the first fire mission does not reduce the position or change the tactical situation sufficiently, other fire missions may be called for.