With Show Comments turned off - no check in checkbox - the red line is a representation of the quantity of comments within the document. The length of the red line corresponds to the number of comments in the document therefore a long red line would indicate a larger amount of comments than a short red line. If a document does not have a red line under its name then there are no comments in that document. Another more visual way to determine if there are no comments in a document is with Show Comments turned on - check in checkbox - and to notice that there are no comment branches from the document.