If the Opening Section boundary includes a circular arc segment (e.g. a curved roof), or is a full circle (e.g. a circular tunnel or shaft), then you can easily create an arc segment or circle while you are using the Add Opening Section option, as follows:
At the point where you wish to begin the arc:
Select Arc from the right-click menu, OR
Type a followed by Enter in the prompt line.
In the prompt line you will then see:
Number of segments in arc <20>:
Press Enter to accept the default value of 20 or type in a new number.
You will then be prompted to enter the second and third arc points. After entering the third point, the arc will be drawn connecting the three points you entered. If you press c and Enter before selecting the third point, the arc will close on the start point of the current polygon.
Enter a point on the circle.
Select Arc from the right click menu or type a followed by Enter in the prompt line.
Input the number of segments for the circle.
Enter a point on the circle diametrically opposite the first point.
Press c and Enter, and you will have drawn a circle with a diameter between the two points.
Note that arcs and circles are actually made up of straight-line segments. It is suggested that the user experiment with this option to see how it works.