Dear Oscar,
that is the material number. you get the maximum physical (Si units) coordinates for each material number. You can see that in the implemenatation
https://www.openlb.net/DoxyGen/html/d5/d9e/classolb_1_1SuperGeometryStatistics2D.html#accd028ebb8f0d9352a87afed0268e7b8
Best
Mathias