Screw Types
Type A For use in joining metals, from .020 inches to .060 inches thick.  The screw has a gimlet point primarily for use in sheet metal, resin impregnated plywood, etc.
Type B With thread pitches somewhat finer than Type "A", includes a blunt point of use in sheet metal, non ferrous castings, plastics, resin plywood, etc.
Type C Thread forming or machine screw has machine screw diameter pitch combinations with thread approximating American National Form. Screw has a blunt tapered point. Used where a machine screw thread is preferable to spaced thread types of screws
Type AB This screw has a gimlet point as used on the "A" type and pitches match those of the "B" type.  Increasingly popular for many applications.