I guess it depends on the processes and technology that are used to make milk protein concentrates, whether or not the chemical composition is altered. Depending on the process, that may not be the case, so that it would be a milk solid. In general, usually the milk protein concentrates are permitted in the cheese products, where they would be used to provide the main function of cheeses, which is the protein aspect.
On June 1st, 2006. See this statement in context.