The RST System is a standard system used for reporting (R)eadability, (S)ignal Strength and (T)one of received signals.
For voice contacts only the “R” and “S” are used. The “S” is the strength as judged by ear and generally is not the same as your S-Meter reading. The RST System was developed in 1934, well before most radios had S-Meters, so few if any S-Meters produce readings that match the RST System.
For CW signals, if the signal has the characteristic steadiness of crystal control, you may add the letter “X” to the RST report. If there is a chirp, the letter “C” may be added to so indicate. Similarly for a click, add “K.” That said, most rigs are crystal controlled, or sound like they are, so adding “X” is generally not done.
Report signal quality honestly! If you report a station as 59 when he's actually 22, he's likely to reply with a lengthy monologue about his station, weather, and hobbies... or in the case of our nets he will give you your signal report only once, and you run the risk of missing it. If you report an accurate 22, the wise person will limit return comments to name, location and a minimum of details, or in the case of our nets a signal report repeated several times. Accurate signal reports facilitate communication.
The minimum acceptable signal report on the 3905CC nets is 21.