Defines a domain of the geometry with constant height (Thickness) in -z-direction. The first Layer in the Extrusion starts by default at z=0 (and can be changed by using the parameter GlobalZ in a LayerInterface). Each subsequent Layer in the Extrusion starts at the -z-position where the previous Layer ends. A DomainIdMapping has to be attributed to each Layer for defining domain identifiers for the different extruded objects in this part of the geometry.