If your boat is stock, then you have two large batteries (4D's). The battery switch determines which battery you want to use at the moment. If you are hooked up to shore power, and the battery charger is turned on, it will charge both batteries. You can verify this by switching the voltmeter between Batt 1 and Batt 2.
When you are motoring, the alternator will only charge the battery you have selected. You could select Both for this function. When anchored, or sailing, I would select 1 or 2 and leave it there. When starting, I would switch to the other battery. If both batts are weak, you could switch to Both for starting. It is not a good idea to leave the switch on Both for an extended period, since one bad battery will then kill both batteries.
Many of us C310 owners have installed a separate start battery, and paralleled the two 4D's to make a more robust "house" battery bank. There are many threads on this forum concerning these upgrades in the archives, and Maine Sail has excellent tutorials on batt switches and maintenance.
If you want to post pictures of your battery compartment and panel switches, along with your sail number, etc., we could better address your specific needs. You have chosen an excellent boat, and I am sure you will enjoy her!