[Porchdog]

I always used to hear that it was any holes larger than a nickle.

Now it's enforced that you can't have holes into the passenger compartment or rust that compromises the structure - so as Fred says, rocker rust on a full framed pickup is OK. Rusted cab corners fail. Rusted rockers on a Toyota Corolla fail (and make it hard to open the doors).

Inspections here are by private garages and there is a lot of variation in the opinions of the inspectors. I know at least one garage that will pass any amount of rust. My guy doesn't want to see any perforation, even in the rockers of a pickup.