VTK grid format/ Boundaries of mesh

I am trying to create a structured grid using vtk in python. The process is straight forward :

  • create points (as numpy array).

  • create cells as list of indices from the points list.

How to add boundaries to the mesh? It does not seem to be written in the file format instruction.pdf

" https://vtk.org/wp-content/uploads/2015/04/file-formats.pdf "