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.

You must input one type of boudnary faces.

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;