It's not a method of classification, it's a way to mark what is classified (and at what level) and what is not. While I have never sent/received classified emai

It's not a method of classification, it's a way to mark what is classified (and at what level) and what is not. While I have never sent/received classified email, in the past, I have dealt with classified Navy message traffic. Markings like (C), (S), and (TS) were "portion marks" and marked subjects, paragraphs, etc. throughout the message. (U) is used to mark portions which are unclassified (in a classified message).