A project's main file is its main document. When a project is opened, the main file will be displayed by default. This file calls the other HTML documents. Usually, it's the file that contains frames (index.html, default.html).
Display the Project Manager.
In the display area, click on the document you want to designate as the Main File.
Click on the Main File icon.
From then on, the Project Manager will display this document in bold characters so that you can easily identify it.