It's done with EPB, so rear only.
So, the manual just says "keeps the brake applied after releasing the brake pedal", which implies it's equivalent to leaving your foot on the brake pedal, except you didn't (for the CVT model).I don't think it's the EPB because I never hear any noise when you start moving again. If it was the EPB the brake hold would turn off very slowly and would be audible. My guess is it uses the ABS controller and/or the brake booster to hold the brake pressure. In that case it would probably be all 4 brakes.