It also represents the two types of electrical current now running thru the bus. DC is battery power that is recharged with solar, while AC is converted into DC from the batteries. It is now also coming in from shore power ( a campground supply, or house, or a generator) which is basically plugging into “the grid”. The big terror, and design headache was – how to integrate two separate circuits into one, with a switch, without spending $1,000 or more. This called for a $70 switch, instead of a $300 switch and a “kit” for RV’s. The switch prevents the shore power from “backfeeding” into a very expensive inverter and melting components.