It is difficult to answer, because the sentence doesn't make sense to me with either option. I can't work out what it is trying to say. Where does ti come from? Is there some more context that you could provide? Generally though, "a" and "the" will work equally well in a sentence. The question is which meaning you intend. Is it a specific hierarchy the has been identified? If so, it will be "the". Is it just any hierarchy? Then it will be "a".