HI Tudor,

re:
> why can't be done a bool operation of a plane and a surface knowing the plane normals ?

That's because MoI is designed to try and not behave differently depending on which direction an open surface happens to have as the "positive" normal direction.

The booleans are based on working with solids. If you are working on all surfaces you should use the Trim command instead of booleans usually.

- Michael