I'm going to argue that the criteria is
- If the question can be answered with an unambiguous answer, then the question is in scope.
- if the answer is opinion related, multivalued or not an answer, then the question is out of scope.
- If the answer is about a primary source, it is in scope; if you ask for help in interpreting a specific artifact, picture, narrative, document, etc., then the question is very likely in scope.
If the answer is a secondary or tertiary source, then the question is probably out of scope; the burden of proof shifts and you should justify why it should be in scope.
If you are looking for the canonical source for any question, then the question is in scope (canonical tends to a single source).
- If you are looking for a specific primary source, then there will be one answer, and the question is in scope.
If you are looking for a good reference on X, or information on Y, then the answer is multivalued and out of scope.
There are some edge conditions that we just have to rely on good will to resolve. If you ask Samuel Russel and I for canonical sources on the source & origin of the great depression in the USA, we're going to refer to different canons; I'm a rabid monetarist with Austrian inclinations; he is a historian specializing in labor theory. We have different understandings of the problem. But there are two important points.
First, when I cite Schecter vs US and he cites some other work, each of us will disagree on the priority & pre-eminence of the work, but we will mutually acknowledge that the other is citing a correct canonical source from their historical perspective. He may say that I'm wrong in my conclusions, but he will (probably) not disagree with the facts, just the interpretation.
Second, and potentially more important, history is a verb - history is a process of creating narrative from sources. We all acknowledge that we bring different perspectives to the table. I continue to cite Mr. Russell because although his perspective is so alien that I almost need a dictionary to understand it, he is always civil and respectful, always focused on the facts, not on ad hominem. So long as we follow his example, we can resolve differences of opinion about whether a source is canonical or not.
Most importantly of all, disputes about canon are an opportunity to discuss the practice of history and to educate - which is I think the core value of H:SE.
I'm going to put my soapbox away now, and hope that it hasn't been resting on anyone's toes while I've been speaking.