Many people that I have encountered seem confused by the "Start Battery" concept, or worse, have no idea what they are doing when they use the battery switch.
I explain that on my boat the start battery is used like the spare tire, I don't use it unless necessary. I normally start the engine and run all of my house loads (lighting, charging phones and small VHF radios, propane solenoid, 12VDC refrigerator) from the house bank. My start battery is automatically maintained by an echo charger, or when I switch the battery switch to "BOTH" when motoring.
I also maintain a schematic diagram of my 12 volt system as a living document that I keep aboard. Here is an older version as a JPG to give you an idea.