That's unusual because it looks like they combined a BEP Marine voltage sensing relay with a Blue Sea manual switch.
Here is the best guess for operation without seeing the actual model numbers.
Use the switch in the 'On' position whenever the boat is off the trailer which will allow the DVSR to automatically isolate the two batteries when voltage drops and combine the two batteries when voltage has returned to normal. This way the starting battery will always be held in reserve for dependable starting after a stay at rest and using your electronics.
Turn the switch to 'Off' when the boat is in storage.
Use 'Combine' in an emergency where you need to manually force the two batteries to parallel.