$U_o$ is the operating phase-to-phase voltage of the system. The range is any positive value between 12 V and 550 kV.
The IEC 60287-1-1 Ed. 2.0 still limits DC systems to 5 kV DC because DC cables have additional constraints such as temperature dependent stress profiles and void formation. The electrical resistance of the insulation is temperature dependent as well as dependent of the elecrical field. And for HVDC cables the temperature drop across the insulation can limit the current rating.