What is the purpose of _hbEvent function definition

I am new to javascript and ran across this function definition on a web page. What is the purpose of the _HBEvent callback? I am not really interested in the code as much as the function _hbEvent definition. What event will cause it to be called???

var _hbEC=0, _hbE=new Array; function _hbEvent(a,b){b=_hbE[_hbEC++]=new Object();
b._N=a;b._C=0;return b;}
var hbx=_hbEvent(“pv”);hbx.vpc=“HBX0200u”;hbx.gn=“www35.vzw.com”;
hbx.lt=“auto”;//LINK TRACKING
hbx.lc=“y”;//Lower Case

There is nothing in the code you provided that makes use of the callback ability. If a callback is not provided then the function just returns undefined, which is what normally happens if no return is provided from a function.