It was created to test the accuracy of the weighting and ensure the index was correct. With consideration to the history of tether's effect on Bitfinex's spot market, I had interest in the differences in Bitfinex's USD and UST pairs. Although the difference now is minor, xxx/ UST pairs in the index are more accurate.
Options to configure are choosing the base currency, BTC or ETH, and the quote currency, USD or UST (USDT).
Sorry if I haven't replied to your message yet, I'm a bit backlogged :)