CloudCompare forum

Remember that
1- Meshing then samplepoints can help to increase pointscloud density (but take care about any convex hull area)
2- 2.5D volume doesn't work if you have more than 2 z values

In the below image, there are 4 z so 2.5D volume doesn't work (manual modification is needed)
..

Thank you very much for taking the time to draw those out and craft an example!

That clears up a great deal for me.