These are familiar to most boaters as a large, usually red switch with a round face and prominent knob. The 4-position selector switch has long been the industry standard for managing the selection and charging of two battery banks. Blue Sea Systems has three families of manual battery switches.
Some switches have an auxiliary contact called an AFD (for Alternator Field Disconnect) which is used to disable the alternator field whenever the switch is turned to an open position. Otherwise, the alternator diodes are susceptible to damage by "cutting" the charge current to the battery.