Floating slab for interior floor. If you place the weight of the walls and roof on it by extending it out, the settlement will happen at the edges causing the slab to bust up in the middle of the floor due to the edges settling and the center not. I purchased a building built that way and it stinks having cracks through center of my building slab from edge to edge in multiple directions. Probably three inches higher in the center of my building than the edges now after 20 years of settlement.