The Smart Meter allows the inverter to know what the total consumption of the house is. With this information, the inverter will be able to send the amount of energy needed to be as close as possible to 0W displayed on the Smart Meter.
However, it is not essential. Without Smart Meter, the inverter will not know the total consumption but will be able to supply all the devices connected to its AC Backup output.
In both cases (with or without Smart Meter), the inverter will be able to supply the consumers up to 3kW for the 3.6 inverter and 9kW for the 9.12 inverter.