Programming question on Outback VFX

On page 11 of the programming manual for the VFX is says:

"low battery cut-out establishes when the FX turns off to avoid draining the battery. If an AC source is available and AC INPUT is set to DROP, the FX will transfer the AC loads on the FX to the AC source. A built-in five-minute delay reduces nuisance FX shutdowns. This setting’s range is between 9.0VDC and 12.0VDC. Press DOWN to view the next INVERTER screen."

What is meant by AC Input set to DROP?? I can't find any other reference in the manual about setting AC Input to DROP. I want to use this function of low battery cut-out set point to allow AC loads to switched from being powered from the batteries through the inverter, to being powered by grid power coming from the AC Input. I just don't understand what is meant by "AC Input set to DROP".