The bottom row of app shortcuts works similarly to the home screen, but can only contain app shortcuts. It also appears the same on all three home screens.

You can remove and rearrange them in the same way as on the home screen. To add an app shortcut to it, I first add an application shortcut to the homescreen, then drag and drop it into the bottom tray. Since the app tray is filled by default, you'll need to remove an app first.