Sold as two bushings.
WFB or Worlds Fastest Bushing- is a special formulation of urethane where there is a lubricant mixed in to the urethane. Which is why these bushings will sometime look a little crusty.
The result is an even deeper turn than the APS allows. So if you are looking for a bushing that is supportive but still allows full controlled deep turns, then throw some of that WFB into the mix.
A lot of riders feel the double barrel (Barrel bushings on road side and board side) give just the right amount of return to center support and break from center feel.