The upper-left pane contains the Assembly, Collaboration, and Views panes by default, but you can add other panes such as BOM and Markers. The lower-left pane contains the Properties and Collaboration panes by default. You can change what tabs are displayed, reorder and undock panes.
To change what tabs are displayed, click Window > Show/Hide, then the pane's name.
To reorder the panes, drag a tab to a new position on either the upper or lower pane.
To undock a pane (make it an independent window), drag a tab out of its pane.