From the manufacturer

SLS (Super Light Steel) springs are now available from FOX for DHX and DHX2 rear shocks. These SLS springs are a lighter weight steel spring option offered in addition to our standard steel springs. The new SLS springs from FOX are lighter than titanium springs at more affordable prices. A proprietary surface treatment and stress relief process removes residual stresses from the spring, allowing them to withstand higher stresses. Each spring is optimized by using a smaller wire diameter and fewer coils, creating a lighter spring.