Currently, the QSA and QRK codes are officially defined in the ITU Radio Regulations 1990, Appendix 13: Miscellaneous Abbreviations and Signals to Be Used in Radiotelegraphy Communications Except in the Maritime Mobile Service.
[8] This format is most notably used by the BBC for receiving signal reports on postcards mailed from listeners, even though that same standard specifies that the SINPFEMO code should be used for radiotelephony transmissions.
An extension of SINPO code, for use in radiotelephony (voice over radio) communications, SINPFEMO is an acronym for Signal, Interference, Noise, Propagation, Frequency of Fading, Depth, Modulation, and Overall.
This format originated with the U.S. military in World War II, and is currently defined by ACP 125 (G).,[11] published by the Combined Communications Electronics Board.
[citation needed] This is because digital signals have fairly consistent quality as the receiver moves away from the transmitter until reaching a threshold distance.
At this threshold point, sometimes called the "digital cliff,"the signal quality takes a severe drop and is lost".
[citation needed] This format is unsuitable for radiotelegraph or radio-telephony use because it focuses on voice-to-noise ratios, for judging whether a particular telephone line is suitable for commercial (paying customer) use, and does not include separate reports for signal strength and voice quality.