What does the values in column clazz (osm2po) mean?

I have a given table in postgres with OSM data. There exist two columns which I don’t understand. They belong to osm2po. They are labeled with “flags” and “clazz”. I know, that flags describe the kind of road. It means, whether it is driveable by car, bike,… The problem now is, that I do not understand the meanings of the values in the column clazz. I just know that they also describe the road type. Can anybody explain, what the values like 12, 15, 31,… mean? Does a documentation exist where the meaning of the values is written down?