The mesh refinement procedure starts from the mesh in the input deck. All elements which are not of type C3D4 or C3D10 are kept and their element numbers are not modified. All other elements are deleted and the gaps in the numbering between the elements which are not tetrahedral are filled from low to high by the newly generated tetrahedral elements. The procedure for the refinement is based on the book by P.-L. George and H. Borouchaki .
The mesh refinement procedure uses a lot of mesh description fields, most of which are dynamically modified during the refinement. These fields are introduced first.