All primitive shapes can be modeled as solid or hollow shapes. This is shown by solid or outline preview in the assets drawer. At the moment of writing we have three shapes that can be both hollow or solid by selection before you start modeling. Both hollow and solid shapes remain stretchable and editable throughout the development of a design
Solid shapes are generally great for modeling and sculpting forms from the outside when you don't want to think much about what is happening to the interior spaces.
Hollow shapes are driven by a default parametric shell thickness (walls, ceiling, and floor) that defaults to 300mm thickness and can be updated in the settings panel using the "wall" slider.
in Arkio v0.6.4. we included a new primitive shape that creates a window filled opening in hollow shapes, this shape looks a bit like a flat hollow shape and can be found in the menu next to the prism shapes.