The Foundation Slab type has 3 more foundation-specific values than a regular structural floor has. These additional values, which are instance parameters displayed in the Properties palette, are: Width, Length, and Elevation at Bottom. Using these values, an analysis tool can distinguish between a standard structural floor and a foundation slab.
Foundation slabs may be used to model structural floors on a grade, which do not require support from other structural elements. Foundation slabs may also be used to model complex foundation shapes that cannot be created using Isolated or Wall Foundation tools.