BESS = Battery Energy Storage System
The question really gets into broad inverter and system design questions. A disadvantage of grounded DC systems is that they must be galvanically isolated (with a transformer) from grounded AC systems, otherwise there will be a direct short through the inverter power electronics. But transformers make inverters less efficient, so manufacturers have developed non-isolated designs, but these inverters must absolutely not have a battery or PV conductor grounded by the installer. These systems use methods broadly similar to GFCI to stop operation if one of the DC conductors becomes unintentionally grounded, since that can lead to exposed parts becoming energized and other dangers.
Without knowing more about the inverter you're using, it's impossible to comment on its specifics. But I'd always go with the manufacturer recommendation. And if it's not an inverter with galvanic isolation, then that's almost certainly a requirement and you have no choice.
There is no general requirement that a BESS be grounded or not grounded, but inverter design will usually determine one or the other, and that cannot be messed with.
