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The voltage rating of a relay is to tell you what voltage to use across the coil to operate the replay. If it is designed to operate on 120vac, then you would probable damage it or burn it out using 230v. This would be the voltage you are applying directly across the coil.
The current rating is telling you how much current the contacts of the replay are rated for. Generally the current rating will be listed at a particular voltage (AC or DC).
The voltage rating has to do with your control voltage. The current rating has to do with the load current of the device that the relay is operating.