These are rounded head screws with a square base according to DIN 603 as well as NFE 27-351. They are particularly suitable for light wood assemblies that are difficult to drill.
The square collar on the underside of the screw head prevents the screw from turning when the nut is removed.
Cup square head screws according to DIN 603 are light and stainless.
The oval head screw is particularly used in industry and construction.