About Fair Use, as I understand matters
I am not a lawyer, and I am certainly not your lawyer. This represents my understanding of the issue, not specific legal advice.
The Fair Use doctrine, which is a concept in US intellectual property law but other nations have broadly-similar ideas in their laws, allows for “nonprofit educational purposes” as a fair use, and answers on Stack Exchange might reasonably qualify for that. Merely being a type of fair use does not automatically make a given instance of copyright infringement OK; the doctrine has four parts:
- the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
- the nature of the copyrighted work;
- the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
- the effect of the use upon the potential market for or value of the copyrighted work.
(17 U.S. Code § 107. Limitations on exclusive rights: Fair use)
Deciding whether or not something qualifies for Fair Use is a subjective process, that can basically only be performed by a judge, in a courtroom, when a suit is brought and Fair Use is brought up as a defense. None of these things is a firm “this is OK, that is not,” they are considerations that the law has directed judges to look at when thinking about potential Fair Use.
In the context of our site, we can look at these four considerations:
Our purpose is educational, and since Stack Exchange does not (currently) serve ads on this site (aside from the ones we make ourselves and that Stack Exchange receives no money for), profit is not really involved. Again, since there isn’t a firm line here, there being some tangential profit or potential future profit is not necessarily damning; newspapers can include quotations for reviews, which helps them sell newspapers (which make money because of the ads in them), and that is an explicit example of fair use.
The nature of the copyrighted works we deal with don’t do us any particular favors here; that’s primarily about limiting copyright protections from protecting data that isn’t really the author’s to protect. The rules text and/or diagrams, the art included in the books, and so on, are all very much the property of their respective owners. But again, these aren’t criteria for Fair Use, they are things to consider.
and 4. The last two are important on this site, though: how much of the work are you quoting, and how is that going to affect the value of the work? Where the first point is basically always in our favor, at least for legitimate answers, and the second is basically not, the majority of the time, meeting the last two is where we can say that one use is OK and another is not. The crucial test here is that your use is not fair if it replaces the original work: if there is no reason to buy the product or otherwise consume the work after seeing your use of it, you are violating the copyright holder’s rights.
In short, the court would likely look favorably on usages that limit themselves to just what they need to make their larger point (educational, in our case), and that don’t unduly affect the value of the original work in the process. Quoting some-but-not-all of the rules for a class, even quoting an entire spell’s or feat’s rules, including one of many of the images in a book, so long as they are justified by improving the educational value of the answer, these things are unlikely to be seen as violating copyright because the book still has so much more left in it of value—you aren’t threatening the original work with these kinds of things.
Ultimately, however, Fair Use is a subjective thing. As I said, it only really gets pinned down if you get sued and make the case in front of a judge; until then you just have your rationale, and the copyright holder may have a different one. Neither is definitively “right” or “wrong” until it gets tested in court. As a matter of practice, copyright infringements are dealt with by the copyright holder sending a cease & desist letter, and then, if that is ignored, possibly a lawsuit. The C&D always starts matters because it is vastly less expensive than a lawsuit.
As far as meta policy is concerned
The real question here is how we should view potential copyright infringement/fair use cases as a matter of policy on this site. Realistically, the risk we face is a C&D forcing us to remove the infringing material, forcing an answer to be reworked to go without it—obviously, we are/Stack Exchange is not going to ignore a C&D if we get one, it’s just not worth it.
Which has been the guiding principle for how this site handles copyright for a long time: we don’t link to pirating sites primarily for this reason. Sure, many of us also have philosophical objections to piracy, but the real point for this site is that infringing sites frequently get C&D’d out of existence, leading to link rot and bad answers. That is the primary thing to avoid as far as this site is concerned.
So any particular use case has to be considered in light of how likely is it to suffer due to a C&D causing a bad answer. That means we need to keep points 3. and 4. in mind when considering a potential use. That means that, if something is over the line, it’s a bad answer because of the risk of damage from a C&D.
And if that’s our policy, the risk of an actual lawsuit is just about negligible. C&D’s happen first and happen more often/more easily; if we are trying to avoid those, we’re all-but-certain to avoid lawsuits.
Not as legal advice, but applying the proposed policy to the proposed use
I would say that using a single diagram from a book is well within the policy guidelines I have proposed. I consider the risk of a C&D for such a use to be quite low. I would feel comfortable with a such a use in one of my own answers.