Not sure what this option is or what it does? Should I have it on or off?
The Eco mode turns the DC power off after several hours being left on with very low power demand. For example if you were charging a phone the AC200 would turn off if put in ECO mode after several hours without any other additional loads. This is a feature that keeps the battery from discharging excessively if you left the DC power on with no or little load. Basically shuts the system down if you forget to turn off the unit off with no or little load.
Basically I turn ECO mode off as I see no use for it myself.