The File - Edit Project command opens the a project editor tab for the selected project file. A browse window is opened which allows you to select a project on your system. A project editor tab is opened and displays the selected project. If an editor tab was already open for the selected project, then that editor project tab is displayed in the session window.
You may also open an edit tab by clicking on the Edit Project icon in the Sonnet task bar, then selecting "Open" from the pop-up menu which appears.