After generating a mesh or Delaunay tetrahedralization, TetGen default outputs the boundary faces or the convex hull into a .face file. However, this file can be omitted by -F switch. If -r switch is used, TetGen can also read the .face file for identifying boundary faces in a reconstructed mesh. The optional column of Boundary markers is suppressed by the -B switch.
If the boundary marker in the first line is '1', each face has an additional boundary marker (an integer) in the last column. Boundary markers of facets are defined in the .poly or the .smesh files. They are tags used mainly to identify which faces of the tetrahedralization are associated with which PLC facet, and to identify which faces occur on a boundary of the tetrahedralization. A common use is to determine where different boundary condition types should be applied on boundary faces.