The “C” in the left or right margin gives notice to the reader that the information contained in that particular paragraph/sentence is “classified”, not that it is classified ‘Confidential.’ Obviously, not every single piece of information in an entire classified document is sensitive, that is why the designator is placed in the margin. However, the entire document has to be classified at the highest level of the highest classified information contained anywhere within the document. There may be various levels of sensitivity of classified information contained in one document.
The "C" does stand for Confidential. Each paragraph is marked with the highest classification contained within that paragraph.