From the publisher's website:
Think of a Storytelling Adventure System product (SAS) as a story kit, as if you’d bought a piece of modern furniture and brought it home in a big flat box. You open it up, eager to be the Storyteller for your troupe, but what you find is a collection of pieces and parts. To put it together, you’ll need some tools: the rules and worlds provided in one or more of our Storytelling game rulebooks. You’ll use these parts and tools to build a story together with your friends. It might not look quite like you expect it to when it’s all done, but as long as everyone enjoys it, it doesn’t matter how you end up using all the pieces, or even if you throw some of them away.
The basic parts that make up most SAS stories are simple: Storyteller characters, scenes and some advice on how you can put them together. Each of them can be used in different ways to keep the story building towards its climactic end. These parts are designed to make the job of being a Storyteller easier, faster and more fun for you. The wondrous game experiences you’ve read about that shock and satisfy your players come from doing a great job, and everything in an SAS product is intended to pick up the slack so you can focus on creating the best story you can.