What are some of the regional terms for such footwear?
USA: flip-flops, although along the east coast they are sometimes referred to as zories and in Texas clam diggers. New Zealand: jandals. Australia: thongs. South Africa: Plakkies and slip-slops (or just slops). South Pacific: go-aheads etc...
How can such footwear be categorised?
As single-plugger or double-plugger, depending on the number of lateral fixation points (per side). The double-plugger is widely regarded as superior. This thong is the more humble single-plugger.
Lateral x-ray of the foot demonstrates a screw through anterior part of a sandle.