Start Points are markers that allow you to specify which Artboard your prototype should start playing from. These are most useful if you’re uploading a Prototype to Sketch Cloud and you want the people you’re sharing it with to know where to begin.
Start Points are defined in the Preview window by clicking on the icon next to an Artboard’s name. When viewing Artboards marked as Start Points, the icon will appear blue. You can also see which Artboards are currently marked as Start Points by revealing the dropdown menu in the Preview window and checking which Artboards have a Start Point icon beside their name.
To remove a Start Point from an Artboard, select it in the drop down menu and click on the icon again. The icon should now appear grey.
It’s likely you would want to define a Start Point at the very beginning of a flow, but you can also define multiple Start Points — a feature that will be very useful when you’re building more complex prototypes that contain many Artboards, or if you only want others to view a particular part of your Prototype.