Self-Tapping screws are meant to be installed directly into sheet metal. Tapping Screws require a pilot hole and tap their own threads during install. They cannot be used with machine thread tapped holes or machine thread nuts.
Self-Drilling screws have a drill-shaped point, eliminates need for pilot hole. All “self-drilling” screws are considered “self-tapping”, but not all self-tapping are self-drilling
