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Is there a policy or guidance concerning criteria moderators should apply before unilaterally closing questions?
Obviously, if a question is blatantly off-topic or has another problem the moderator would be justified in closing it.
However, in a few rare cases I have seen moderators unilaterally closing questions with no vote even though the question was potentially valid. For example, in one case the moderator commented "I'm not sure this is on topic." So, the moderator explicitly said they were unsure whether it was a valid question, nevertheless they closed the topic without allowing a vote.