It signifies maximum RMS AC voltage that cable insulation can withstand continuously. 600V between any insulated conductor and earth (metal covering of the cable), 1000V between any two phase conductors. Take a look at IEC 60227-1
I agree with your explanation but I am afraid this is not the IEC correct number of standard.
From IHS Standards Store: IEC 60227-1 Document Information: Title:
Polyvinyl chloride insulated cables of rated voltages up to and including 450/750 V – Part 1: General requirements.
I think the correct number may be: IEC 60502-1 Power cables with extruded insulation and their accessories for rated voltages from 1 kV (Um = 1,2 kV) up to 30 kV (Um = 36 kV) – ch. 4.1 Rated voltages.