You may need to make holes in Slabs to accommodate objects such as staircases, or for an open atrium space.
1.Select the existing Slab.
First Select the Existing Slab
2.With the Slab tool active, draw a new contour within the Slab’s boundaries.
3.This new contour will be interpreted as a hole in the Slab.
See also Selecting Slab Hole.