Difference between Rated Load & Max. Switching Voltage on Relay Contacts
Rated load is essentially how much power can be carried through your relay (P = V*I = 300W). You can have a higher voltage if the current is lowered or higher current at lower voltage. This is the first limitation of your relay. If you don't respect it, your relay will simply burn out.

The second limitation is that voltage inside the relay could arc and provide a connexion even if the relay is not clamped. There is a limit point of voltage before an arc is created through thin air (air got decent insulating properties.) Therefore, voltage that pass through the relay should not exceed that limit. Otherwise, you risk damaging your relay.

The switching limit is not the max voltage required to toggle the relay! Do not confuse that data with the usually switching voltage at the control port (3,3v 5v 12v 24v typically).