What suspension modifications need a cert?

Are there any objective rules around what exactly requires a cert? For example I can go to Repco and buy an aftermarket shock to replace my worn shocks, and that doesn't require a cert. Presumably I can replace sway bars and other chassis bracing with no issues as well. I always hear that coilovers require a cert, but what about cars that have coil-over-strut suspension from the factory? Can I install replace my worn factory coilover components with aftermarket coilovers without needing a cert?