KiCad design rules modify copper pours

This is pretty neat – I specified a 4mm spacing between A* and Default net classes:

(rule "HV A* to Earth"
        (constraint clearance (min 4mm))
        (condition "A.NetClass == 'A*' && B.NetClass == 'Default'"))

And then after re-running copper pours, it cleared out the bottom copper pour to create clearance from H3/TP6 and the copper pour:

This is super neat as space around vias and such in planes will automatically have the right clearance.

Some resources on the custom rules engine:

The real power of the design rules is the wildcard matching. If you name your net classes intelligently from general->specific, then you can match just parts of the name to cover general cases and then drill down as needed.