Why I can’t make a boolean difference (aka subtraction) on those two surface objects? I would like to subtract the green pipe from the violet surface. I know it`s a basic question. Those surfaces are only for my learning purposes.
boolean_difference.3dm (1.4 MB)
wrong command, try trim instead. 
Thanks. It worked but I had to change view into wireframe mode. What that “!” sign means. There is no tooltip.
It shows you the likely problem area that is causing the Boolean operation to fail.
Hi -
The ! is a Dot object and has secondary text. Select the dot and open the Properties > Dot panel to read this secondary text.
-wim