Interface between astguiclient and asterisk

What is the method astguiclient used to communicate with asterisk, say making a phone call? I couldn't get a clue when read the sour code. Thanks.
VICIDIAL astGUIclient discussion forum
http://forum.eflo.net/VICIDIALforum/
http://forum.eflo.net/VICIDIALforum/viewtopic.php?f=4&t=4234