Self drilling screws and self tapping screws are frequently mixed up by global buyers. The most obvious difference lies in the tip structure.
Self drilling screws have a drill bit shaped point. It can drill through thin steel sheet without pre-drilling, then tap threads at the same time. They are standard fasteners for metal roofing, light steel frames and steel cladding, often assembled with EPDM rubber washers for waterproofing.
Self tapping screws do not have drill tip. You need to drill pilot holes first. Its sharp thread cuts thread inside pre-made holes. Self tapping screws are more common for wood, plastic and thin metal sheet.
Choose self-drilling screw if you need direct drilling on metal; pick self tapping screw when pilot holes are available.