TargetMC query

Hi, I may be being dense here (i am a newbie) but

Could someone please explain to me why the ‘targetMC’ (variable?) used throughout does not have to be defined…

*The first line is initiated when the the a file begins to load in the targeted mc (which is later set to “container”).

*… when and how is it associated with ‘container’
http://www.kirupa.com/developer/actionscript/moviecliploader2.htm