Shell Inside Outside in Fusion 360

The SHELL command lets you choose the direction of the shelling action - INSIDE or OUTSIDE. If you choose INSIDE then it puts the walls along the inside of the existing shape and maintains the existing exterior surface shape.

If you choose OUTSIDE then Fusion will add a wall outside of the existing shape and the original geometry will become the hollow interior. This is useful for making casting molds or to maintain clearance when designing cases and enclosures.

To understand the difference between the two options, start with