It sets how many days before a delivery date's cut-off the choose your meals reminder email goes out to subscribers who still have unselected meals. The default is 3. Sprwt works out each delivery date's cut-off from your closing-days/cut-off settings, subtracts this number of days, and the scheduled reminder job emails customers whose selection window falls in that range, telling them how many meals are still to choose and linking to the meal-selection page.