The sUSD pool was born out of a partnership between Curve and Synthetix who seeked to help bring stability to their stable coin sUSD. The pool is not a lending pool which means your main APY only comes from trading fees. The pool has sUSD, DAI, USDC and USDT. Unlike Y pool, the sUSD pool is quite cheap to deposit in making it a good choice if you want to try Curve with a small amount.