Sodium Metabisulphite: Food additive or processing aid in potato production?

I work for a fresh produce manufacturer and distributor in Canada and am looking for insight on using sodium metabisulphite in peeled potato production. As per, Health Canada guidelines, sodium metabisulphite (SMBS) can be used as a food additive in unstandardized foods not exceeding 500 ppm. However, I have had many discussions with my peers as to whether SMBS is a food additive or a processing aid in our production. In Canada, a "food additive" means any substance in which the use of which results, or may reasonably be expected to result, in it or its by-products becoming a part of or affecting the characteristics of a food. In our facility, SMBS is used as a preservative and an anti-browning agent to pro long the shelf life of the peeled potatoes. The process is that once peeled, the potatoes pass through a tank on a conveyor with a SMBS solution and are air dried and packaged for distribution. Would this use of SMBS be considered a processing aid or food additive? As a follow up question, if considered a food additive is the 500ppm limit the concentration max in the rinse solution containing SMBS or in the final potato product? 

Thank you!