Faces that will be operated by a Direct Modeling operations are defined in the following UF_MODL_dfo_region_t structure.
"boundary_faces" are optional, and you may specify more than one seed faces. If there is no boundary faces,
the seed faces will be processed. Otherwise, the extracted region between the first seed face and boundary
faces, as well as other seed faces, will be operated.

All blends adjacent to the extracted region will be automatically recognized. If users want to, in purpose, treat some of them as not blends,
they can store these faces in the "excluded_faces" array. This is however not encouraged.

For sheet body, the region cannot be faces on the sheet body boundary.