I saw the claim that an appeal to authority was an "ad hominem". This contradicts how I have always seen the two terms used. I am interested in if there is a tradition in "argument classification" where an "appeal to authority" will be described as an ad-hominem. I thought this Stackexchange might be a good place to ask.
Technically speaking, the term 'ad hominem' means 'to the person', and applies whenever someone tries to inject personal attributes of a speaker into arguments about the value of a position that the speaker holds. It's normally used with respect to negative attributions: e.g., "We shouldn't listen to Person X's views because Person X is a [derogatory term]." However, it is also correct to use it with respect to positive attributions. For a silly example, if someone says: "We should believe Person Y's views on astrophysics because Y is a kind and honest person," That is clearly a type of ad hominem. Being kind and honest has no relation whatsoever to astrophysical knowledge; the attributions are added merely to inject Y's personal characteristics into an otherwise impersonal subject.
There's always a judgement call on ad hominems; sometimes a person's attributes do have a direct impact on the quality of their argument. One should be suspicious of the views of someone who habitually lies, and one should give credit to the views of someone with (say) an academic degree that relates to the topic. But if someone appeals to authority as mere authority — this person has power, and thus must be right — that would certainly qualify as an ad hominem in the positive sense.
The first thing you need to know is that there is no universally accepted canon of fallacy. You'll find there's a lot of wiggle room based on context and interpretation. Consider the following argument.
P1: The outcome of the experiment is subject to my expertise in science alone.
P2: So you're saying no one is free to interpret the results to reach a conclusion?
P1: As president of the science club at high school, I am clearly the most qualified, am I not?
P2: You have the authority to buy t-shirts, and certainly a passion for science, but perhaps this lab experiment we are conducting might be understood by others.
P1: I don't believe so, because you are a stupid freshman who isn't the president of the science club and a senior with a near perfect ACT score.
Now, in the last statement by P1, are the words fundamentally an ad hominem or a fallacious appeal to authority? I don't believe it's quite clear. Calling someone stupid is clearly ad hominem, but is pointing out that someone is a freshman? And is it really a bad appeal to authority if you stack up a 13-year old against an 18-year old with some form of credentials and talent? While this example is written from the cuff, it's the sort of messy real world rhetorical exchanges that make it difficult to put arguments in tidy boxes.
The computer scientist C.L. Hamblin in his Fallacies says in the chapter "Arguments 'AD' that Locke recognizes the term predates him, and says that it goes all the way back to Aristotle. (p. 161) He quotes Aristotle:
...these persons direct their solutions against the man, not his argument.
So, while the term itself is Latin, the recognition of the fallacy itself is ancient. Note how simple the criterion is in this quotation. I'm not sure that modern definitions are that much more precise.
Let's take a more contemporary source. T. Edward Damer has Attacking Faulty Reasoning in which he introduces his ARG schema. A claim must be acceptable, relevant, and on good grounds. He also produces categories. He puts the fallacies in two distinction categories. Abusive ad hominem is a fallacy of irrelevance. Irrelevant or questionable authority is a fallacy of irrelevant appeal. He of course provides some insight into his categories in the respective chapter.
So, what can we take from the difficulty in answering your question?
A) It's questionable that there is some sort of consensus or canon on classifying informal logical fallacies.
B) From A, we can infer that such a cause is likely to be the ambiguity that is inherent in natural language itself. Diagnosing bad reasoning in rhetoric is not like doing so in mathematical logic.
C) From B, even with a work like Toumlin's Uses of Argument which provides us for a language for dealing with rhetorical argument, still nothing is added to classifying fallacies.
Thus, given the inherent ambiguities of language, and taken with the fact that a claim can have all sorts of semantic interpretations and complexity, there's not canon in regards to classification. WP in fact recognizes this fact:
There is controversy both concerning whether a given argument really constitutes a fallacy in all of its instances and concerning how the different fallacies should be grouped together into categories.
My personal experience is the same. I've answered a number of questions about fallacies on this site, and sometimes claims do not provide the basis for a clear label because they contain multiple elements, or they are phrased in unusual ways. Probably not what you want to hear if you were hoping for something that resembles Linnaeus's binomial taxonomy, but natural language is a tricky thing.