Start points 
let you 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. We’ll also automatically create a start point when you add your first Link.
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 unselect the Set as prototype start point checkbox in the PROTOTYPE tab of the Inspector.
The Set as prototype start point option for an Artboard is on the PROTOTYPING tab of the Inspector in the Mac app (v92 September 2022)
Setting multiple Start points
You can set more than one Start Point to guide people through different aspects of more complex designs (for example, a sign-up flow and a user settings flow). 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.