From Serious Sam Wiki
The Viewport is the central editing element of the Serious Editor. It projects map related data in the current view for fast editing access.
The Viewport is mainly used for
- examining the current status of the project,
- editing position, rotation, stretch, range, and target of displayed items (Viewport editing),
- selecting displayed items to show/edit their properties in the Properties group (Properties group editing),
- applying tools to displayed items (Command bar group editing).
For more information see