Energy Meter, Modbus Card, and Modbus Server are optional accessories specifically used for MPI hybrid series inverters only.  The Modbus card provides an interface which acts as a liaison between the inverter and the Energy Meter, so users can monitor the level of energy that is both imported from and exported to the grid.  Modbus server is an additional hardware required when users need to combine multiple units of inverters, such as in a 3-phase configuration, where there are multiple Modbus Cards present.