According to the standard, C13/C14 plugs don't have a set voltage (unlike something like a L6-20P), so it is up to the consumer/manufacturer to decide which voltages are appropriate.
I recently stumbled upon a C13/C14 cable that had "10A 125V~" imprinted on the C13/C14 connectors. The cable itself was rated for 300V and was 18AWG.
My question is, what exactly determines the voltage rating (in this case 125V) for the connector portion of the cable? Would it be considered "safe" to use this in a 208V circuit (many C13/C14 cables are commonly used in datacenters to supply 208V).