Typically, a subfloor is installed over floor joists, but for slab-on-grade construction, sometimes a subfloor is installed atop the slab—such as when installing wood flooring. In such a case, it is wise to insulate between the slab and the subflooring.
The insulation can be rigid foam board or rockwool and usually involves sleepers. In the case of this project, the slab is a slabless slab: compacted gravel and there are no sleepers.
Two layers of subfloor are offset and glued together, forming a floating subfloor over a floating layer of insulation.