The makings on the switch are not very helpful, but it should be described in the caravan's owners manual if you have one.
The standard fridges used in UK specification caravans can be powered in three different ways, from LPG, mains 230Vac or 12Vdc.
The 12Vdc cooling option is wired up to only run when the caravan is coupled to the tow vehicle, and it's alternator is producing power.
When uncoupled on site the fridge is wired so it cannot access the caravans 12Vdc to cool, you are limited to use mains 230Vac or the caravans gas supply, but depending on the model of fridge, it may need some 12Vdc from the caravan to run it's control system and display.