I agree the diode is probably there to isolate one part of the DC system from another, but in the example you mention i.e. a problem with the battery (short circuit etc) the battery bus wouldn’t immediately be affected (unless there were larger problems) as, assuming standby power switch is in auto and you have AC power, the battery bus is powered by TR3 and isolated from the battery via the open battery bus alternate relay. Any problem with the battery would probably manifest itself with popped hot battery bus, battery charger and maybe inverter remote and SPCU CBs (and maybe bat discharge master caution). If you didn’t have AC power then battery bus normal relay (connecting TR3 to battery bus) would be open, battery bus alternate relay would be closed and battery bus would get power directly from the battery. You could, however, get other switching issues affecting the battery bus if you lose the SPCU (which is on the battery side of the DC SYSTEM) as you lose some control of these relays I believe.