It's all about insulation and current capacity. I'm not sure about all
cases, but I think the normal voltage rating for any wire used in
house wiring (I can't say if that's true for extension cords,
appliance cords, tool cords) is around 600V,

"Junior" hard service cord (the "J" in SJT) means 300v. That is still OK for
240. The better cords like SO are 600v.