This field in the Bin Label report (RBL) is only available when you select the Promotion File as the "File to Use." The purpose of this field is to allow you to add RBL to a daily queue to print labels for promotions whose start date is X number of days away. For example, a daily RBL using the Promotion file with ‘Days before Start’ set to 1 will print labels for promotions whose start date is tomorrow.