Otherwise yes, strictly speaking you would only ask for a citation as evidence of some claim. But even then, people would still use it informally as a stand-in for "I disagree," "prove it," "says who?," and so on.
It was never my intention to start a meme like this. I started [citation needed] because specifically Wikipedia was (and I guess still is) replete with assertions that had no evidence. I got increasingly annoyed at half-baked statements that could not be easily removed, either because they sounded plausible or because someone would object.
So I decided to create the {{fact}} tag, which would highlight the phrase that had the questionable content. That way, the article could keep the material, but it would be easier to a. signal to the reader that the statement's veracity was in doubt until some sort of citation was provided, or b. someone (hopefully the original author!) would provide a citation for the statement.
It was, to put it mildly, wildly successful. It appears to have been my greatest contribution to Wikipedia, and, it appears, to wider society.
Not sure how this makes me feel. I was hoping for something more significant, but I suppose that if I have contributed something worthwhile then this might as well be it!
http://knowyourmeme.com/memes/citation-needed
Otherwise yes, strictly speaking you would only ask for a citation as evidence of some claim. But even then, people would still use it informally as a stand-in for "I disagree," "prove it," "says who?," and so on.