If the intended designated use of a drug is for “prevention” versus for “treatment”, how is the population estimated in each case?

If the drug is being used as a preventative, the population estimate should be the number of people to whom the drug will be administered annually in the United States.
If the drug is being used for a treatment, the population is comprised of the number of persons in the United States who have been diagnosed as having the disease or condition at the time of the submission of the request for orphan drug designation