The caption area at the top of the application window contains a customizable Quick Access toolbar:
Dark Theme: 
Light Theme: 
This toolbar is always visible and provides convenient access to the most often used tools.
By default, the Quick Access Toolbar has 6 tools. These tools are also on various tabs on the Ribbon Toolbar, but are placed here for convenience so you can easily:
• Save a project
• Open a project
• Select Items
• Undo / Redo tool
• Open 3D View tool
• Show Help