What is the difference beteween Structural slab vs slab on grade?

Structural slabs are typically elevated concrete slabs - 2nd floor, 3rd floor, roof, etc. These slabs will contain steel reinforcing (rebar) or post-tensioning (tendons). The steel provides strength to support loads, ductility to prevent sudden failure, control of movements/cracks (due to temperature changes (thermal expansion/contraction) and shrinkage stresses (which occur as concrete cures)). Slabs on grade are concrete slabs cast on the ground, usually over an aggregate base course. They may or may not contain rebar or tendons (sometimes they contain welded wire fabric) - these are all for movement/crack control, and may also be used where soil problems are anticipated (expansive soils). There is also something called a structural slab on grade (or structural slab over backfill). A common place to see these is on exterior slabs that occur over backfills adjacent to the basement of a building. This artificially compacted backfill will often settle, especially if exposed to water (even if properly compacted). If it is critical to limit settlement of a slab that is over backfill, the slab is usually designed and constructed as a structural concrete slab which spans across the backfill soil (and does not depend on support from the backfill soil). The slab will usually span from the basement wall (which it bears on or is connected to), over the backfill soil, and bear on native soil.