I have struggled to find clear guidelines as to what is an acceptable flag or not and this is a good explanation.
However for new users things need to be a bit clearer than: "for the good of the platform".
New users with significant online followings often repost their content to Steem as a way to get started and then get flagged for plagarism. @steemcleaners method of dealing with this situation is less than ideal, especially when these users have often been censored on the very platforms steemcleaners is using to verify identity.
The precise expectations for referencing sources is also not clear. If you link to an image (rather than copy it) do you need to explicitly reference as well given the reference is in the link?
Is is acceptable to downvote just because you disagree with the content? - I had thought not, but some people do it.
There is no guideline.
People will literally flag you on here because they didn't like your face. Or flag you because they are in an alcoholic rage. The list is endless.
I, for one, preserve the flag for when I see things that may be illegal, spam, plagiarism, lies or scams.
Cheetah/SteemCleaners will comment a few times before flagging for duplicate content giving plenty of time to verify with them that the duplicate content is your own site. Cheetah has one job and it's very narrow, find duplicate content.
As for flagging because you disagree, while I feel you are free to use your stake as you choose, but if you say you like blue and I like black I don't feel that's a good reason to flag. If you said you liked blue and voted it for $100 and I disagreed the reward pool should be used in that fashion to that extent, I'd be comfortable flagging it.