A GUI Status is created in an ABAP Program using the statement: SET PF-STATUS ‘<Gui Status Name>’.
A GUI Status is contains a Menu Bar, Standard toolbar, Application toolbar and function key settings. All of these elements combine to define the look and feel that will be associated with the user interface for the ABAP program.
An ABAP program can have multiple GUi Statuses associated with it. A Developer can create their own GUI Statuses, use existing GUI Statuses or a combination of Both.
If no GUI Status is defined in the program then a standard user interface is displayed by default.