Screws and bolts come in a wide range of head shape, head type and threading combinations. When it comes to head type, there are a few common options. The flat head screw is tapered on its underside so it sits flush against the material it's driven into. Oval head screws are similar to flat head screws, but they're a bit more decorative. Both pan head and round head screws are meant to protrude above the material they're screwed into, and they accommodate washers the best. Hex head, or hex cap screws differ from the others because they require a special driver to be installed; they're used when extra torque is needed.
If you look at the head of a hex cap screw, you might notice lettering or dash marks. These markings are like a code to designate the screw's specifications, such as what standards it meets, what it's made of and how much of a load it can bear. For instance, if you see a hex cap screw with three evenly spaced marks that start at the edge and point toward the center, that means the screw is made of medium carbon steel that's been quenched and tempered. Depending on the size of the screw, it has a minimum tensile strength of 90,000 to 120,000 pounds per square inch (psi). Hex cap screws with six dashes, each of which radiates from an angle toward the letters PB at the center, are Grade 8 screws made of medium carbon alloy steel. They have a minimum tensile strength of 150,000 psi.
For the sake of consistency, manufacturers are required to include these markings on their products so that everyone in the industry is on the same page. Plus, manufacturers have to include some kind of marking on their screws that indicate who the manufacturer was.