Before revision: Most indirect hexahedral meshing methods rely on 10 patterns of subdivision of the hexahedron into tetrahedra . A recent observation at least one more pattern exists raise the question of the actual number of subdivisions  of the hexahedron into tetrahedra.
After revision: Most indirect hexahedral meshing methods rely on 10 patterns of subdivision of the hexahedron into tetrahedra are there? It is well known that a cube has five subdivisions into 6 tetrahedra and one subdivision into 5 tetrahedra. However, all hexahedra are not cubes and moving vertex positions increases the number of subdivisions of the hexahedron into tetrahedra.