For the region of faces, we have two ways to allow you input boundary faces.
You can either input the individual boundary faces in bndary_faces array or
you can input boundary faces as rules in bndary_face_data array.

The boundary data is optional. You can leave both type of boundary data to
be NULL.

The input rule data for boundary faces have to be one of the
UF_SC_FACE__data_p_t types.

If bndary_faces is not NULL, bndary_face_data array will be ignored. The
bndary_face_data will take effect only if num_bndary_faces = 0 and
bndary_faces = NULL;