Can be used on 2D and 3D views to select all elements based on the selection you have chosen on the Select toolbar. The Select Wall, Space, End, Window, or Shade option must be selected otherwise this option will be greyed out on the Edit menu and you won't be able to select it. The keyboard shortcut for this command is Ctrl+A.
Example
The Select Wall button is highlighted. Left-click anywhere in the 2D drawing area to shift the focus and set it to receive input. On the Edit menu, click Select All. All internal and external walls will be displayed in red and black depending on their alignment. If the wall alignment is set to snap to the left or right of the wall then the walls will be highlighted with a single red and black line. Otherwise the walls will be drawn with a black line centred between two red lines. If the focus then shifts to the 3D window then the command will work with everything except for the Select End tool.