The term "functionally grounded" was created because the PV industry is dead afraid of labeling DC PV conductors as ungrounded. If we pick it apart a functionally grounded system is an ungrounded DC system with a way to test the DC conductors to determine if any of them have become grounded and if they have then shut down the system. Remember the GFDI fuses in the neutral to ground bond that we said for years made a DC system solidly grounded? They are now considered a functionally grounded system.