In Editor, a specific screen can be designated as a "Start Screen", which is the screen that will be launched when a link to a project is clicked. From Editor, right-click on a screen in the Screens panel and select 'Set as Start Screen':

It also designates which screen will be used for the project thumbnail shown in Definition Center's Project List and Project Detail pages. 

When the "Start Screen" is unspecified the first screen in the Screens panel is used as the "Start Screen" by default.