The CSK Head Screw, commonly referred to as the Countersunk Head Screw has a "flat-head". And this type of screw has an unusual head that dips into the surface and rests flush in materials.
They are called countersink because they sink into the object they are used for. The flat head also tapers along the shaft into the material.
You could turn the screwdriver anti-clockwise to remove the fastener in the material or you could use a rubber band with rough powders.
They can be used for Timber, wood, steel, and board applications.
The style of a screw's head is referred to as being countersunk or non-countersunk. Countersunk heads have tapered edges that enable them to be pushed deeper into the surface of a workpiece. A non-countersunk head type has no tapering, which causes the head to protrude.
The measurement is taken under the head to the end of the fastener in CSK head screws, where the head typically rests above the surface.