The SSEG is the inverter.
Fill in one row per system - there would usually only be one.
Strike out whichever of New/Existing doesn't apply. If there is already a system on site and you are adding to it, then you will have two rows, one Existing and one New.
If it is single phase then only the Phase 1 column needs to be filled in. Enter the kW output rating of the inverter. If it is 3-phase then fill in the 2nd and 3rd columns as well. G83/2 requires that 3-phase systems are balanced, so you should have the same value in each column.
Type test ref is the G83/2 test reference for the inverter. Usually a copy of the test certificate is available on the manufacturer's website and there will be a number on that to use.
That last column is self-explanatory.
