In normal Amplitude Modulation (AM) signal generation a carrier and two side bands are produced. The side bands are designated Upper Side Band (USB) and Lower Side Band (LSB). It is actually not necessary to transmit all these signals to provide the receiver with enough information to reconstruct the original modulation, audio in this case. The carrier may be removed or attenuated, and so can one of the two sidebands. The resulting signals will require less transmitted power and will occupy less bandwidth.
USB is the norm for aeronautical HF communications, AM is used when a carrier is required such as while being checked for maintenance and as far as I am aware LSB is unused by acft.