Particularly low grade, highly contentious or excessively vague ingredient we usually recommend avoiding
Special ingredients in dog food
Legally, UK dog food producers are obliged to state roughly what is in their products. However, if divulging some of the ingredients is likely to result in a competitive disadvantage - for example, if it gives away a companies 'secret formula', this obligation can be waived.
The problem for dog owners is that terms like 'special ingredients' could refer to literally anything, which makes choosing the right food very difficult. As always, where ingredients are unclear, we would recommend assuming the worst and looking elsewhere.