Indicates whether a Window is main.
When you start an application, the first Window displayed is a main Window. It represents a central Window that provides common application functionalities. Visually, it differs from other windows, in that it has a navigation control. This navigation control is an Action Container for the ViewNavigation Action. It lists items corresponding to declared business object types. End-users can use these items to open List or Detail Views that allow them to browse and edit the corresponding business objects.