16"=16 inches and 16'=16 feet. "Slab to slab" refers to the dimensions (nagasa, hirosa, takasa etc) something is being measured from one concrete slab (usually, but not always, the foundation) to another concrete slab. In this case i suspect that the dimensions should be 16' (feet) instead and that they are measuring the distance between one wall and another because both the walls and the foundation can be made from concrete slabs. The essential meaning is that the ceiling is 16 feet wide or long when measured from one wall to the opposite wall. HTH.