BOPP film classification: eligibility for concessional customs duty is prospective and confined to the amended tariff entry. An explanation added to a tariff notification that defines plain plastic film to include Biaxially Oriented Polypropylene (BOPP) film has prospective effect and applies only to the specific tariff entry where it was inserted; the amendment under one entry does not extend to another entry absent an express insertion, and BOPP eligibility under the other entry arises only from the date that entry was separately amended.