Each type of radio emission is classified according to its bandwidth, method of modulation, nature of the modulating signal, and type of information transmitted on the carrier signal.
An emission designation is of the form BBBB 123 45, where BBBB is the bandwidth of the signal, 1 is a letter indicating the type of modulation used of the main carrier (not including any subcarriers which is why FM stereo is F8E and not D8E), 2 is a digit representing the type of modulating signal again of the main carrier, 3 is a letter corresponding to the type of information transmitted, 4 is a letter indicating the practical details of the transmitted information, and 5 is a letter that represents the method of multiplexing.
A similar designation system had been in use under prior Radio Regulations.
The letter occupies the position normally used for a decimal point, and indicates what unit of frequency is used to express the bandwidth.
There is some overlap in signal types, so a transmission might legitimately be described by two or more designators.