I understand the house, emerge parallel, and start switches; but what is the switch to the very left of the panel and when is it used?
The fourth switch is a service disconnect switch and allows you to disconnect the house batteries from powering the inverter. You would only need to turn this one off if you were servicing the inverter. Your charger and inverter go through that switch. With the switch in off position, both the inverter and charger are NOT connected to the battery.