These are most regularly used as seat bolts in karts, or anywhere where clearance over the head of the bolt might be an issue. (They're also slightly less likely to rattle loose due to the additional surface area of the countersunk face, though given that you're almost certainly using nyloc nuts everywhere on your kart, it probably doesn't matter... so there's a totally useless fact for you!).
The length of these is measured over the entire length of the bolt, including the head. They have a 5mm hex socket.