It's supporting nothing other than it's allowing the generator run at a higher rpm for more power. For max charging under these circumstance. Now if I was to use something drawing more than set amps then the charging would stop till load was done. I personally only use the generator for charging when needed.
Under my charging parameters its taking nothing from batteries. Your getting bogged down in the wording "load support" by Magnum.
Manual...
Standby Mode:
When an external source of AC power (i.e., shorepower or generator) is connected and
qualifi ed on the inverter’s AC input, it operates in Standby mode. In Standby mode, an internal AC transfer relay automatically closes to pass the incoming AC power directly to the inverter’s output to continue powering the connected electrical appliances. The unit is also monitoring the AC input and “standing-by” to switch to Inverter mode if the unit detects a power failure (or low AC voltage). While in the Standby mode, the unit can operate:
a) as a battery charger, which converts the incoming AC power into DC power to recharge
the batteries; or
b) in load support, when the incoming AC source is not large enough to power the inverter loads by itself, additional current from the battery is used to ensure the inverter loads continue to be powered.