converter

Folks, I have a brand new, 2020 Hawk Shell (yay) with furnace and a single battery. We got the shell because we wanted to have the furniture and galley just they way we like it, even though FWC does a great job. However, the Shell doesn't come with a shore power option, so I've got a new DLS30...

New 30 amp Battery converter/ charger iota dls-30/IQ4 for sale. not needed as I changed over to lithium based system. Specs: https://www.iotaengineering.com/dls30.htm $125.00

First question - What is the function of the 3-way rocker switch on the front panel next to the thermostat? I'd assumed it was on the 12v circuit to source select battery-off-shore power. But that can't be it - today I took off the front panel because I was confused... now I'm not - there is...