SelCal (Selective Calling) uses two pairs of codes (originally Motorola tuned reed relays similar to the ones Ma Bell used to use to switch long distance calls). The 4 letter codes are assigned by your country's ICAO representative usually to a specific airframe but some operators such as OEMs may have common company code for test purposes.
The ground station transmits the code in two simultaneous bursts - say AB + CD - and, if you're tuned to that frequency and it's your code, you'll get a chime and possibly an EICAS message or light. Common installations have a total of up to five SelCal channels - 3 VHF & 2 HF is the most I've seen.
The main advantage is that, especially on HF on long hauls, you don't have to have the squelch so low to hear anything that the white noise gets to you although modern HFs are much better. BTW there are scanning (or at least frequency-hopping) military HFs.