A project can create as many windows as it likes. The Window Manager maintains the windows for the project. Once a window is created using make_window, the project can load data into it (with load_item_in_frame) and set its properties. The window does NOT have to be visible in order to load items into it.
There is a default window called 'system' that is defined by the Test Engine. This is the window used to display the billboard and the about window. You can use this window for your project, but if the user selects to show the about window, then your content will be replaced.
The Window Manager on Windows uses a main MDI window with user-defined windows as MDI client windows. There is a default window called 'application' that is defined by the Test Engine on Windows. You can use most of the window functions to set its properties. Other platforms may not have this window, so use is_window ('application') to determine whether the window exists before accessing it.
The MacOS uses standard floating windows and there is no 'application' window defined.