Why The RST system's third digit rates tone, and letter suffixes refine that tone description. A "C" appended to the report (for example 579C) tells the sender their signal chirps, meaning the frequency shifts slightly each time the key is closed, usually from an oscillator or power supply being pulled by keying. Other traditional suffixes are "K" for key clicks and "X" for a crystal-stable note.
Watch out Key clicks, the harsh clicking from too-fast keying rise and fall times, are reported with a "K" suffix, not a "C"; a report of 100 percent copy is already carried by the R (readability) digit of 5.
C for Chirp, K for Klicks, X for Xtal-stable. The letter matches the first sound of the fault.