Flat Head Screw

Flat screw head sits entirely with the same plane of the mating surface. It has a countersunk head with a flat top surface and cone shape under the bearing surface having 82 degree standard.
The degree of a flat head denotes to the angle from the topmost of the head to the surface where threaded part join the head.

One can use it where protrusion (bump) is unacceptable above mating surface, for example if you are building your bookshelf, you don’t have to worry about your screw coming out from the surface and grasping other stuffs on it. Also, used in steel application, door hinges etc.