Openings

 Enables you to insert a 2D / 3D opening into a wall, roof, or roof slab representing a window, door, etc.

...

Note: To insert actual "smart" windows and doors, see see Windows and Doors

...

In this tool, each opening is represented by two blocks - a 2D and 3D view, so that the block can be seen in both orthographic and isometric views. The blocks must be created in advance before they can be inserted into a wall. See Creating a Block.
The blocks do not have to be the same thickness as the walls, roofs, or roof slabs they will be inserted into.