It would appear that on your particular model it has no function, here's what a review at TBreak has to say "There’s also an expansion slot below the optical drive that in other models can be used to equip a specially designed UPS, which gives you a few precious moments in a power cut to save all your work before the system goes into hibernation." found here http://tbreak.ae/asus-g10ac-review/
I checked to see if that UPS was available for your model and found nothing