The Hexa-Dominant option will generate a hexa-dominant mesh using a bottom-up meshing approach. The Hexa-Dominant mesher starts with surface quad dominant mesh and uses an Advancing Front scheme to fill as much of the volume as possible. For simple volumes, it can fill it completely. For more complicated volumes, it usually fills several layers in from the surface with hexa elements and then fills the middle with tetras and pyramids. Then a diagnostic is run, and if those central elements are poor, the inner volume will be meshed again with the Delaunay mesher. The surface mesh will be created with the parameters defined under Global Mesh Setup or Surface Mesh Setup. The volume mesh will then be generated from this surface mesh. If a surface mesh exists, you can also choose to specify Existing Mesh for the Input.