Not only the flag but also the close reason is utterly useless by itself. If you cannot be bothered to leave a comment explaining why you think the question is unclear or which further details are needed, do not vote or flag, period. You must clarify yourself what is missing. Maybe more implicit knowledge must be made explicit. Maybe you misunderstood.
If you do not spell it out, the question will at least receive a canned comment leading precisely nowhere, or, worse, remain closed forever. Others cannot look inside your head, they do not know what’s wrong/missing.
Thank you.
Recent examples (not closed, but voted on)