Flatten vs Flatten to Foundation

Can anyone quickly run through the differences between "flatten to foundation" option and "flatten to surface" option?

I never use the foundation one when I want a flat piece of land.
Should I?
Flatten to Foundation - flattens horizontally to where the center of the circle is. This would be used to create new levels of ground either above or below the current plane you are on. Surface will flatten to what ever angle the circle is at...diagonal, vertical, etc. This would be used to make cliff sides, angle slopes, etc.
Only if you want to make a wall of terrain and want it to be flat you use flatten to surface.

Flatten to foundation is just make the land flat.
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