I missed that!
In some High voltage design it is necessary to impose a large clearance for high voltage nets.
Normally a surface distance have to be considered, like 2.5mm for 250V isolation.
But when the same nets run in some inner layer the distance is not anymore on the surface, but insted in volume.
In volume situation the distance for the same 250V isolation is much less (like .5mm) allowing to pack much more tracks in the same space.
In CS I saw that there is a "Net and Layer" rule that can be used for this purpose, but nothing related to "Net Class".
If on a circuit there are tens or hundredths of nets in the High voltage class I need to type a roule for each net and for each layer.
This may easily add up to thousandths of rules...
Is there a simpler way?