he "D" section steel palisade pales strength is derived from the corrugations formed in their cross section during the rolling process, the more defined the corrugations the stronger the pale.
The 3mm general purpose "D" section pale is generally fixed to the rails with 8mm saddle head bolts with permacone (shear) nuts.
The 3.9mm security "D" section pale is generally fixed to the rails with 8mm anti vandal rivets and collars (requiring specialist tools to set) or 8mm saddle head bolts with permacone (shear) nuts.