This is a power bank that supports multiple power levels. Apparently this device can take a charge from variable power source, and can deliver variable negotiable power later to a USB device. Unfortunately the producer completely fails to indicate which negotiation standard is supported. Without knowing this and having corresponding charger and consumer devices, the higher voltages are useless, and the power bank will deliver only default 5 V and 2A, a fairly ordinary parameters.
Currently the advanced charger technology offers two major methods of power negotiations, Power Delivery (promoted by USB alliance), and QuickCharge, a proprietary method introduced by Qualcomm.
The Power Delivery offers four "normative profiles", 5V at 3-5A, 9V at 3A, 15V at 3A, and 20V.
Qualcomm QC method offers variable profiles, but the set of 5 - 9 - 12 was the original one.
Therefore, from listed specifications (5/9/12V), I would guess that this powerbank supports an early version of QuickCharge, so to use it at its full potential you would need a QC charger to charge it up, and your tablet/phones should support QC as well.