Reply To: How to understand vti files
This is the compressed VTK data (storing everything in raw XML would significantly increase the file size). Why do you want to parse the VTK file manually?
If you want to access the material numbers for further processing in your program, the straight forward way is to access the SuperGeometry directly.
If you want to do further post-processing of VTK output outside of OpenLB, I would suggest to check whether there is a VTK library / binding available for your language of choice.