RST stands for Readability, Signal Strength and Tone. On CW all three codes are used for a report such as 599 which means perfectly readable, extremely strong, and perfect tone. On phone only the first two codes are used. A report might be 55 meaning perfectly readable, fairly good signals. In operation the signal strength is often taken from a reading on the S-meter.
The RST System
2–Barely readable, occasional words distinguishable.
3–Readable with considerable difficulty.
4–Readable with practically no difficulty.
|1–Faint signals, barely perceptible.
2–Very weak signals.
5–Fairly good signals.
7–Moderately strong signals.
9–Extremely strong signals.
|1–Sixty cycle a.c or less, very rough and broad.
2–Very rough a.c., very harsh and broad.
3–Rough a.c. tone, rectified but not filtered.
4–Rough note, some trace of filtering.
5–Filtered rectified a.c. but strongly ripple-modulated.
6–Filtered tone, definite trace of ripple modulation.
7–Near pure tone, trace of ripple modulation.
8–Near perfect tone, slight trace of modulation.
9–Perfect tone, no trace of ripple or modulation of any kind.