With Start Points you can choose which Artboard your prototype starts with when you preview it.
Setting a start point
To set a Start Point, open your prototype in the Preview window, select an Artboard from the drop-down menu, and click . You can also set a Start Point for a prototype in the Prototype tab of the Inspector by selecting the Set as prototype start point checkbox. Sketch will also automatically create a Start Point when you add your first Link.
Using Start Points
Whenever you preview your prototype, you’ll always start with the Artboard you’ve set as your Start Point.
To remove a Start Point, select the Artboard in the Preview window and click again, or deselect the Set as prototype start point checkbox in the Prototype tab of the Inspector.
Setting multiple Start Points
You can set more than one Start Point to guide your users through different aspects of more complex designs, such as a sign-up or user settings flow, for example.
Every time you set a new Start Point, it’ll appear as a different prototype in the same document in the web app or the iPhone app.