The Addonics documentation leaves out some critical words that would clear up the situation. In their "Specifications" link they state:
Maximum # of storage devices supported - 16 (requires combination of USB hubs or Port Multipliers)
What they mean is:
The maximum number of storage devices supported on the NAS 4.0's two eSATA/USB 2.0 combination connectors is 16. However, to use this number of stroage devices requires you to add either additional USB hubs or to use eSATAp port mulipliers.
If you attach the storage devices to a USB hub, then they connect via the USB 2.0 feature of the combo connectors. If you use SATA port multipliers then the SATA devices connect via the eSATA feature of the combo connectors.
Note that storage devices here can be either SATA or other types such as "disk on key". Any "disk on key" devices would connect via USB hubs. SATA devices could connect either through additional USB HUBs or through SATA port multipliers.
The critical missing word before "port multiplier" in the Addonics documentation is "SATA". A SATA port multiplier allows you to attach multiple SATA devices to a single SATA host port, at the cost of some performance. See the Wikipedia article on SATA port multipliers.