A region is created when faces form a fully closed volume. As soon as a region is present you can define the properties inside (e.g. set medium to some dielectric). The solver (MoM as default) will then use the Surface Equivalence Principle (SEP) to represent the region. I.e. the faces do behave as if there'a volume, without actually havin to mesh the volume. A surface mesh is sufficient. Note, one can also use Volume Equivalence Principle (VEP) instead of SEP. In that case the volume is discretized in tetrahedras. If not using MoM, with FEM geometry is also meshed with tetrahedras, with FDTD the whole problem is discretized with voxels (cuboids).
In your model, the substrate is modelled with panar Greens functions instead of a volume. That's a very efficient method for printed structures, because the substrate won't require a mesh.
In my graph you se the real part of the input impedance of VoltageSource1.
De-embedding unfortunately isn't supported directly in FEKO. It's under development but unclear when it will be available.