Arguments from authority are inferences from the assertion of an expert to the (likely) truth of that assertion. That is, if an expert says that p, then it is probably true that p.
To be a strong argument, the authority in an argument from authority must really be an expert in the subject matter in question. But how do we tell which expert is the relevant or appropriate expert to turn to with a particular question?
For example, if we want to settle a question in physics, should we turn to a biophysicist or an astrophysicist? If we are wondering about a question in chemistry, do we ask a biochemist or an electrochemist? In other words, how fine-grained does the appeal to authority have to be for it to be a strong argument?