Setting custom boundaries
In addition to the standard boundary definitions included with App Builder, you can make your own custom boundary settings wherever you want them.
App Builder adds the new boundary marker to the horizontal ruler in your project. You can now use this marker to help define and test the various boundary settings for your project.
Note: Alternatively, you can simply double-click in the horizontal ruler at the point where you want to create the boundary. If you use this option, when the boundary marker is created, App Builder automatically moves the right edge of the canvas area to that point.