When you develop a large application consisting of numerous source directories, it is common to split up your code into separate projects. Of these projects, one is typically the entry point for your application and contains the application's main class.
To tell the IDE which of your projects is the main entry point for your application, you set one project to be the main project. The IDE provides commands that act on the main project. For example, running the Build Main Project command builds both the main project and all of its required projects, thereby ensuring that you all of your compiled classes are up-to-date. Only one project can be the main project at any time.
After you set a project as the main project the keyboard shortcuts for Run (F6), Build (F11) and Clean and Build (Shift+F11) will apply to the main project regardless of which project is selected in the Projects window. Keyboard shortcuts for Debug and Profile will also apply to the main project.
To make a project the main project:
Alternatively, you can right-click the project node in the Projects window and choose Set as Main Project.