I just bought some frozen chicken breasts, and on the box it says "18% meat protein."

Does that mean there's some unnatural additives? Or does it just mean that for every 100 grams of chicken I'll get 18 grams of protein?

Also, is there any reason I should be worried about eating pre-marinated frozen chicken breasts? It would make my life a lot easier in terms of preparation, but I'm not sure whether there's any arguments against the nutritional status of those pre-frozen foods.

Thanks! :)