Socket-Head series (socket-screws or Allen-head screws) screws are recessed-drive screws, most typically with hexagon drive but also made with Torx, Bristol and other styles. They are made in in various head-types and commonly used in toolmaking and in the construction of precision machines etc. where access, head-size, or strength require the special features of these high-grade precision engineers screws. Black-oxide is the most common finsh but zinc and nickel etc are often applied.
Sholder-screws have ground diameters and a reduced thread size.