What's the difference between "appeal to unqualified authority" and "ad hominem circumstantial"

Dr.Bo will give the correct answer, I will try to see if I explain it correctly. Ad Hom Cir, is simply claiming that because a person has a vested interest in something "THEY MUST" be lying about it, not that they actually are lying. So it's fallacious to claim the person is wrong because they have an interest in "outcome" even if they are right. Unqualified authority is someone using someone else who "Cannot be considered an authority" on a subject but they appeal to the person as if their opinion is from knowledge of a subject. See the difference?
appeal to authority.......
Person A says that X is true
Person A says Person B agrees with X as being true and Person B is an authority on X
Therefore X is true
Person B isn't an actual authority....
Ad Hom Cir....
Person A claims X
Person B claims Person A has a vested interest in X being a particular way
Therefore, Person A must be lying about X.....
Person A could actually be telling the truth about X but it's being argued as a lie simply because of them having an interest in the outcome.
ok, gave it a shot

Thus, because Johnston had a clear motive to lie, we should not believe his statements.

This appears to be a clear ad hominem circumstantial because the wording "we should not believe his statements" implies, to me, that his statements are necessarily invalid because of his motivations. Good communication is about being clear and removing as much ambiguity as possible. I would have stated as

Thus, because Johnston had a clear motive to lie, we should be skeptical of his statements.

Here, were are not suggesting he is necessarily lying, but that there is a good chance he is. This makes all the difference.

The appeal to unqualified authority is something quite different. The unqualified authority does not have to have a motive for being dishonest—they simply have to be unqualified in the topic.

Doesn't the inclusion of "unqualified" authority present a Slippery Slope factor to the question, when an appeal to authority argument implies that the authority in question may be outside his area of expertise or simply irrelevant?

Example:

So and so does not wear socks, when challenged about it he appeals to an authority by responding
Einstein didn't wear socks. This is a classic Appeal to Authority fallacy.

By inserting the descriptor Unqualified we open up a whole other can of worms. When arguing the inerrancy of the Bible's account of creation, Young World creationists argue that one should not rely upon Physical Theorists or secular cosmologists to explain Biblical creationism, but instead turn to so-called authorities in creationism for a better understanding. Circular reasoning aside, the argument sets up a false equivalency by assuming that secular cosmologists are unable to explain creationist theories of the Bible. In one sense it is correct to suggest that scientific cosmologists are unqualified to explain creationist theories. On the other hand, it is a clear distraction and deception. Unfortunately, the argument now diverts to who is qualified and who isn't. A Cosmologist may be perfectly qualified to argue the scientific accounts of cosmological origins, but unqualified to expound upon the specifics of religious pseudoscience. He or she might be able to refute the claims without knowing the dogmatic precepts behind it

While a Physical Theorist may not be an authority in Biblical creationism, they are very qualified to refute the Creationist worldview. By inserting unqualified into the mix the argument deceptively diverts to a subjective argument about qualification.

Without wishing to suggest I know more than Hurley, based purely on what you've quoted above, I can see why you're confused.

The example given does present an Appeal to Unqualified Authority, in that it implies Johnston's position within the tobacco industry is sufficient to believe his statement about the addictive qualities of tobacco.

The refutation to that should point out that Johnston is not in fact an authority on the health implications of tobacco (or at least that he has not been shown to be so). We can assume he knows about packaging it and selling it, but assuming he knows about the medical implications is unwise.

In stating that we shouldn't believe Johnston's statements because he had a clear motive to lie, the author, Hurley, is in fact committing an Ad Hominem Circumstantial fallacy in my opinion. It's true that Johnston has a motive to lie, and this means we should be skeptical and seek further evidence, but it doesn't automatically mean he's not telling the truth.

The two fallacies aren't connected or ambiguous - one is contained in the example (and then poorly refuted) the other is committed by the author.