The other widely used system is CTCSS. The system may also be referred to asd subaudible tones or PL tones (a Motorola trademark). As the name suggests it uses subaudible tones (below about 250 Hz) to carry the selection information. These are transmitted in addition to the normal voice channel, but as they appear below the normal audio range they are filtered out and they are not heard.
Sorry voor het engels maar asd is dus een andere term voor CTCSS