Relay's contact capacity

The Voltage rating tells you about the maximum Voltage you can apply irrespective of the current , and the current rating tells you about maximum current through closed contact ( when there is no voltage across the contacts ) irrespective of initial voltage .

Some big contactor manufacturers mention KW (VA) ratings on contactors along with normal current and voltage ratings. I think in this case you can safely use for 110V but can not exeed current.

To qualify the above, all of them worthy responses, it needs to be stated that these are the manufacturer's recommended maxima for current and voltage. Exceeding them may cause deterioration ranging from 'absolutely no effect' to 'total destruction of the device' in ways that have not been investigated, assessed or quantified. The ratings are therefore conservative and intended for the long-term survival of the device, which may be tested and rated by them for >10⁵ operations, perhaps.

Closing this particular contact at 250V/0.3A, for example, might have very little effect, or it might go bang and start smoking, and therefore the user is outside the manufacturer's recommended comfort zone for the device. If in doubt, substitute a device with a higher rating that encompasses the intended range of currents and voltages to be anticipated in the circuit.