The relative ratio of two voltage levels is given in dBr (decibel
relative). This dB value has no fixed reference value, but results solely from the ratio of the two level values to each other. For example, if the ratio of two voltages
is 100:1, then the dBr value is 40. The relative dBr level value is used for transmission lines or for amplifiers, among other things. It is a planning value that is used to calculate the modulation and which, when added to dBm0, gives the absolute power level dBm: dBm = dBr + dBm0.