If not deemed helpful flags can be declined or disputed. What flags should be declined and what flags should be disputed?
If your flag is dismissed by somebody, it might be for one of several reasons:
This list is incomplete of course
You could have simply misunderstood a post. For example, an answer might not seem to answer the question, but it does if read it more carefully
A seemingly duplicate question might have merit if the differences are subtle but important (think Python 3 questions, or Compiz vs Mutter)
The reviewer took care of the post in some other way. E.g. a down-vote instead of deleting the post, where the flag indicates something unnecessary
The user invalidated your objection by improving their post
If you object to a reviewers decision, you can always just leave a message to one of the moderators, and/or start a discussion about the disputed post in the chat room.
We use our best judgement when approving or declining flags. If you disagree, help us do a better job by discussing the flag specifically.
This is covered in the privilege pages:
It's pretty straight forward, the scope of the site is listed in the FAQ so it's not like we're making judgement calls all the time.
Though every once in a while I might mark a flag as invalid when someone flags an answer that might be not-very-high-quality but isn't so bad that it should be deleted. In those cases IMO it should just be a downvote. I usually decline the flag and then read the answer and it usually just needs a down vote.