Exterior slab.

If this exterior slab is for a patio or something similar, you should be
able to pour the concrete directly on the ground.

as long as your soil has good drainage like the reply poster.

For my driveway and patio, I dug out several inches of impermeable 'clay
like' soil, put down "3/4 minus" road base gravel then used a plate
compactor on that, then formed, added rebar & poured. Came out good.