Our Tannersville to Tuxedo Single Side Band Test (April 16-17 2021) has revealed some variability in the way participants are reporting on the readability and signal strength of transmissions they are able to hear. Below is a quick review. [Source: ARRL and Wickipedia]
The R-S-T system is used by amateur radio operators, shortwave listeners and other radio hobbyists to exchange information about the quality of a radio signal being received. The code is a three-digit number, with one digit each for conveying an assessment of the signal's readability, strength and tone (only used when sending morse code) was developed in 1934 by Amateur radio operator Arthur W. Braaten, W2BSR.
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.
Tone (only used with Morse Code)
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.
If the signal has the characteristic steadiness of crystal control, 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. The above reporting system is used on both cw and voice, leaving out the "tone" report on voice. [http://www.arrl.org/quick-reference-operating-aids].