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, KiCad 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 and I assume can vary based on requirements. This is super useful for high voltage designs.