by John Shannon, K3WWP
(From his web site here)
The RST system is a means of quickly telling the other station a wealth of information with just 3 numbers. When you say to someone 'RST 469', they immediately know you are having a little trouble copying them (4), they know you are hearing them at a good level (6), and their signals are in good shape with no hum or buzz (9). These 3 numbers are independent of one another, and simply because a station is very weak, you can still be copying perfectly, and his tone can still be perfect. This would be an RST of 529. I have heard some stations improperly giving out RST's of 555 or 577 when the other station had a perfect tone. They somehow think the T should track with the S, and if the sig is weak, the T number should be lower. This is completely wrong.
Nowadays, virtually every signal you hear is a T9. The meanings of T1-T8 are throwbacks to the early days of radio when you really could hear a T1 signal. Oh, occasionally you will hear a station with some hum, buzz, or ripple, and should give them a T7 or T8.
In addition to the three numbers, once in a while a letter is added to the end of the 3 numbers. These letters are: X meaning the signal is perfectly steady like a crystal controlled signal; C meaning the signal sounds chirpy as the frequency varies slightly with keying; and K meaning the signal has clicks. X is a throwback to the early days of radio when such steady signals were rare. Today most all signals could be given an X and it is hardly ever used. It is helpful if you hear a chirpy or clicky signal to use the C or K, e.g. 579C or 579K.
The following table lists the values and definitions of the R, S, and T.