Ok, that is for charging other USB devices per link:
"It’s not only the looks of this beautiful Power Keyboard case that make this case desirable, it also features an integrated 4,000 mAh Lithium-ion battery and a built-in USB port, which allow you to charge almost any device via a USB cable while you’re on the go."
Mini USB port for charging the built in battery, full size USB for charging other devices (including the iPad).
No data transfer capability.