Raised Concrete patio construction
The fill is not intended to support the new concrete slab indefinitely. It’s merely fill for the initial concrete pour. The fill will settle overtime, regardless of how well it’s compacted. 

Therefore, the slab becomes a “structural slab” and the reinforcing steel MUST be placed exactly correct, or it will fail. That is to say, it won’t just crack, but it will crack so severely it will fail. 

We all know the reinforcing needs to be on the tension side (bottom) of the slab, but the slab could have “reverse bending” at the edges and the rebar will need to be on the top of the slab. (Occasionally the slab will be designed with the rebar in the center, in order to eliminate the need to transfer the rebar to the opposite side .)

How thick is the slab and how much rebar is called for in each direction?