And I think Michael T was on the right track there with his first reply since he did use shell in there as well.

The difference is that if you want the "side wall" parts to also follow the surface normal then you'll want the side walls to also be generated as part of the shell as well.

If the side walls come from a boolean with an extrusion then those areas will have the one uniform directional quality of an extrusion rather than the "follow the surface shape" type of result that shelling a surface fragment will give you.

Hope that make sense!

- Michael