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In transmission engineering, most often it would be rather impractical to characterize the magnitude of signals directly by a numerical value in volts or watts. Instead, a logarithmic measure is used, expressed in "dB", to characterize the signal magnitude in relation to some chosen reference value. Designations commonly used are "power level difference", "voltage level difference", etc., all expressed in "dB". A level difference from a standard situation is described simply as "level". Loss and gain are also measured in "dB".

The term "relative levels" has been a very useful term in transmission planning for the last 40 years and is expected to be so in the future. However, the public switched telephone networks have changed considerably in these years. Especially with the introduction of digital exchanges which causes some uncertainty concerning the application of relative levels and necessitates some changes in the traditional way of applying relative levels. In this Recommendation, relative levels and associated terms are explained and examples are shown to clarify these concepts.

Guidance on the use of decibels in the field of sound transmission and radio frequencies can be found in [ITU-R V.574-4].

Notations for expressing the reference of a level can be found in Part 5 of [IEC 60027-3].