Unfortunately, our experience with voluntary codes is not a happy one. In the financial services area, for example, there are a number of voluntary codes. Unfortunately, voluntary codes are, as their name implies, voluntary. They include little or nothing in the way of enforcement measures, so people comply only if they want to comply. Sometimes, that creates much more confusion than anything else, because, often, the people who should be applying the codes have no idea that they exist.
I will let François answer your question on labelling GMOs, because he has more practical experience in the area.