Without a primary le the IDE compiles/runs the source of the active window when a program is started. If a primary le is speci ed, the IDE compiles/runs always this source, even if another source window is active. With the menu item "Compile|Primary le..." a le dialog can be opened where the primary le can be selected. Only the menu item "Compile|Compile" compiles still the active window, this is useful if a large project is being edited, and only the syntax of the current source should be checked.
The menu item "Compiler|Clear primary le" restores the default behaviour of the IDE, i.e. the 'compile' and 'run' commands apply to the active window.