Truma Gas/Electric Heater

I am new to caravanning and have used our caravan just the once for a weekend. One thing I failed to get to grips with was the successful operation of the Truma blown air heating on the electric setting, i.e. there is a dial on the heater unit numbered 1 to 5, what does that do? and in which position do I put the little sliding switch located next to the aforementioned dial?