A flat head screw has, unsurprisingly, a flat head. This allows for a flush surface after fastening, which is extremely useful for very low clearance applications. The head is conically shaped, so it will fit into a cone shaped hole cut into the material it is securing. (See Reference 1) The angle of the cone should be anywhere from 90 to 92 degrees. The diameter of the head is anywhere from 9.78 to 10 millimeters. The maximum diameter of the countersunk hole is 11.2 millimeters.
The shank, or shaft diameter should be anywhere from 4.82 to 5 millimeters. No length is specified, since these bolts can be configured to a variety of lengths. The thread pitch is .8 millimeters. The pitch refers to how much distance there is from thread to thread. (See Reference 1)
A flat head bolt can have a variety of drive configurations. These include regular screwdriver slots, Phillips heads, or hex drives for Allen wrenches. For a hex drive, the size specified is a 3 millimeter Allen wrench. (See Reference 1)