Regions are existing configured areas on the main custom module window where you can add your view models. A list of all the available shell regions can be found in the Enum ShellRegions folder.
The UX toolkit currently supports three regions, the Header Control Region, the Main Region, and the Footer Control Region.
To show a view in a particular region, you must use the Modules Controller class. A good example is the ShowClientView() on the SPFClientController. This sets up two views; one view in the Header Control Region and one in the Main Region.
Private Sub ShowClientView()
Me.ShowViewInRegion(Of SPFClientHeaderView)(ShellRegions.HeaderControlRegion)
Me.ShowViewInRegion(Of SPFClientView)(ShellRegions.MainRegion)
End Sub