Appeal to authority (as a fallacy) refers to advancing the opinion of a prominent figure for something on which they themselves are not an authority. Citing Schneier on computer security is not an appeal to authority. Appeal to authority is citing his opinion on the best way to prepare coffee.
> Appeal to authority (as a fallacy) refers to advancing the opinion of a prominent figure for something on which they themselves are not an authority.
This is incorrect. An appeal to authority is when an argument is claimed to be true because someone of authority has stated it. It is a fallacy when the authority of the person has nothing to do with whether the argument is true or not. For example, if a maths authority makes a maths claim, that doesn't mean their claim is true since their authority in maths doesn't determine the validity of their claim. The truth of their claim is determined by looking at the maths itself.
> Citing Schneier on computer security is not an appeal to authority.
It could be, if you're arguing that it's true because Schneier said it. Obviously something relating to computer security isn't true because Schneier says so.
That said, appeals to authority are somewhat necessary in discourse, since we don't have the time to explore all arguments on their own merits. I believe that claims that Schneier has made in the past are truthful, and therefore conclude that I can place some trust in other claims he makes without fully exploring them, but that trust may be misplaced.
Appeal to authority would be something like, "North Korea wasn't behind the attack because Schneier says so." To avoid an appeal to authority, you cite Schneier but make use of his arguments. "According to Schneier, North Korea wasn't behind the attack because if North Korea was behind the attack a different locale would have been used."
The fallacy still applies when the authority gives an opinion on some topic they are knowledgeable about; the fallacy isn't about the relevance of the authority so much as the use of the appeal.
I think that's a simplification. You can appeal to a en expert in A about something regarding A and still be considered an appeal to authority fallacy --that is because 'authorities' can make bad judgments, have biases, etc.
