What are appropriate questions for a closed-door (non-public) part of a PhD (or master's) defense?
Generally speaking anything and everything is on the table unless you are told otherwise. There are no limits. It is up to individuals what is asked and up to those and other individuals to evaluate the answers. The candidate may not be able to answer everything.
But it probably isn't required to answer everything. What is required is to say sensible things, including &quot;I never studied that&quot;.
But for the foundational knowledge of your field, you'd better be able to give pretty good if not absolutely ideal answers.
The (second) worst story I ever heard, though it may be apocryphal, is one in which the candidate in biology or chemistry gave his dissertation presentation in which he repeatedly mentioned pH. There was an outside member of the English department on the committee who said she didn't have any real questions, not understanding the subject, but would like some layperson's explanation of pH from the candidate. The person, who had long ago studied it formally and was able to use, froze up and gave no answer. He failed.
But maybe that was just a horror story told around grad students to scare them into working harder, something like a campfire ghost story.
In a private oral exam of my own I was asked a question and started to develop an answer (math - algebraic topology) and it went nowhere. I stopped and admitted that I wouldn't be able to finish it, but was able to say exactly what the flaw in my argument was. This actually impressed the examiners more than if I'd given the correct answer immediately, as they told me later.
So, you don't have to be perfect, but you do have to make sense. Do that and you will probably be fine, subject to the vagaries of personality.