Charging from the main engines means you are
charging from the alternators, which output DC.
The shorepower or generator means an AC source, i.e., EITHER the land based shorepower OR your generator) which then goes to a "shorepower" (sic)
charger with AC input and DC output.
They are two completely separate methods of charging.
Since there is no AC input
power when you have the generator off and are not at a
dock, it doesn't matter what position that switch is in.