There is no identified regulatory definition of chemical processing of which I am aware. If I had to define it, I would use the definition in Wikipedia for chemical reaction:

"Chemical reaction is a process that results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants. Chemical reactions are characterized by a chemical change, and they yield one or more products which are, in general, different from the reactants. Classically, chemical reactions encompass changes that strictly involve the motion of electrons in the forming and breaking of chemical bonds, although the general concept of a chemical reaction, in particular the notion of a chemical equation, is applicable to transformations of elementary particles, as well as nuclear reactions.

Many different chemical reactions are used in combinations in chemical synthesis in order to get a desired product. In biochemistry, series of chemical reactions form metabolic pathways, since straight synthesis of a product would be energetically impossible in conditions within a cell. Chemical reactions are also divided into organic reactions and inorganic reactions."

Compact it is not. Accurate, it seems to be. I believe that one can develop an accurate definition of chemical processing from there. The other place you can look is in the NAICS (North American Industrial Classification System) for chemical processing. That's a pretty specific list of the information available.