It depends upon what part of the process isin question.
We are going to assume that the Gantt Chartis developed by the Contractor to planconstruction operations.
We are going to assume that the scheduledata are generated sometime before theonset of actual field operations.
Theoretically, S-Curves can be used to control the planning phase as well as the construction phase of the process. As a matter of practice it is rarely done. That is not to say that on mega projects it would not be a good idea.