Typically the two banks consist of a house bank for loads such as lighting
and radios and computers
and convenience items, and a starting bank for starting the engine
The switch should be selected to the house bank when you're aboard using 12V power for house loads.
Shift it to the starting battery
for starting the engine
With the engine running, if you desire to switch back to the house bank, switch it through the BOTH position to avoid de-energizing the field circuit on your alternator
You don't mention the characteristics of your shore charger
, but typical chargers can charge both banks. An AC circuit breaker should be installed inboard of your shore connection to protect your boat if an AC system short occurs. If you have such a breaker it should be shut to energize your shore charger, as well as the isolation breaker for the charger if you have one. If your shore charger is not capable of and connected to charge both banks simultaneously, you'll want to switch the charger output to each bank using the battery
selector switch to charge it. Some people place both banks in parallel by selecting BOTH in order to charge, but there are pros and cons to doing so...not least of which may be that your charger does not have adequate output to efficiently charge both batteries.