On 18 December 2018 16:48:30 GMT+01:00, Karl Rupp <rupp@iue.tuwien.ac.at> wrote:

I have not quickly found how is that "VTK ordering" defined but I 
hopefully it's a well-defined unambiguous cell-local numbering. I will 
try to find it out soon and get back to you.

Hope this helps:
https://www.vtk.org/wp-content/uploads/2015/04/file-formats.pdf
(page 9)

Best regards,
Karli